I am using Visual Studio Express 2012 to develop this application for a w7 x64 platform.
I using the following class to manipulate mixer volume for my application, however I also need to change the system volume.
Code:
Imports System.Runtime.InteropServices

Public Class AppVol
#Region "Constants"
    Private Const MMSYSERR_NOERROR As Integer = 0
    Private Const MAXPNAMELEN As Integer = 32
    Private Const MIXER_LONG_NAME_CHARS As Integer = 64
    Private Const MIXER_SHORT_NAME_CHARS As Integer = 16
    Private Const MIXERCONTROL_CT_CLASS_FADER As Integer = &H50000000
    Private Const MIXERCONTROL_CT_UNITS_UNSIGNED As Integer = &H30000
    Private Const MIXERCONTROL_CT_UNITS_BOOLEAN As Integer = &H10000
    Private Const MIXERCONTROL_CT_CLASS_SWITCH As Integer = &H20000000
    Private Const MIXERLINE_COMPONENTTYPE_DST_FIRST As Integer = &H0&
    Private Const MIXERLINE_COMPONENTTYPE_DST_SPEAKERS As Integer = (MIXERLINE_COMPONENTTYPE_DST_FIRST + 4)
    Private Const MIXERCONTROL_CONTROLTYPE_FADER As Integer = (MIXERCONTROL_CT_CLASS_FADER Or MIXERCONTROL_CT_UNITS_UNSIGNED)
    Private Const MIXERCONTROL_CONTROLTYPE_VOLUME As Integer = (MIXERCONTROL_CONTROLTYPE_FADER + 1)
    Private Const MIXER_GETLINEINFOF_COMPONENTTYPE As Integer = &H3&
    Private Const MIXER_GETLINECONTROLSF_ONEBYTYPE As Integer = &H2
    Private Const MIXERCONTROL_CONTROLTYPE_BASS As Integer = (MIXERCONTROL_CONTROLTYPE_FADER + 2)
    Private Const MIXERCONTROL_CONTROLTYPE_TREBLE As Integer = (MIXERCONTROL_CONTROLTYPE_FADER + 3)
    Private Const MIXERCONTROL_CONTROLTYPE_EQUALIZER As Integer = (MIXERCONTROL_CONTROLTYPE_FADER + 4)
    Private Const MIXERCONTROL_CONTROLTYPE_BOOLEAN As Integer = (MIXERCONTROL_CT_CLASS_SWITCH Or MIXERCONTROL_CT_UNITS_BOOLEAN)
    Private Const MIXERCONTROL_CONTROLTYPE_MUTE As Integer = (MIXERCONTROL_CONTROLTYPE_BOOLEAN + 2)
#End Region
#Region "Structs"
    <StructLayout(LayoutKind.Sequential)> _
    Private Structure MIXERCONTROL
        <FieldOffset(0)> Public cbStruct As Integer           '  size in Byte of MIXERCONTROL
        <FieldOffset(4)> Public dwControlID As Integer        '  unique control id for mixer device
        <FieldOffset(8)> Public dwControlType As Integer      '  MIXERCONTROL_CONTROLTYPE_xxx
        <FieldOffset(12)> Public fdwControl As Integer         '  MIXERCONTROL_CONTROLF_xxx
        <FieldOffset(16)> Public cMultipleItems As Integer     '  if MIXERCONTROL_CONTROLF_MULTIPLE set
        <FieldOffset(20), MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst:=MIXER_SHORT_NAME_CHARS)> Public szShortName As String ' * MIXER_SHORT_NAME_CHARS  ' short name of control
        <FieldOffset(36), MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst:=MIXER_LONG_NAME_CHARS)> Public szName As String '  * MIXER_LONG_NAME_CHARS ' Integer name of control
        <FieldOffset(100)> Public lMinimum As Integer           '  Minimum value
        <FieldOffset(104)> Public lMaximum As Integer           '  Maximum value
        <FieldOffset(108), MarshalAs(UnmanagedType.ByValArray, SizeConst:=11, ArraySubType:=UnmanagedType.AsAny)> Public reserved() As Integer      '  reserved structure space
    End Structure
    <StructLayout(LayoutKind.Sequential)> _
    Private Structure MIXERCONTROLDETAILS
        <FieldOffset(0)> Public cbStruct As Integer       '  size in Byte of MIXERCONTROLDETAILS
        <FieldOffset(4)> Public dwControlID As Integer    '  control id to get/set details on
        <FieldOffset(8)> Public cChannels As Integer      '  number of channels in paDetails array
        <FieldOffset(12)> Public item As Integer           '  hwndOwner or cMultipleItems
        <FieldOffset(16)> Public cbDetails As Integer      '  size of _one_ details_XX struct
        <FieldOffset(20)> Public paDetails As IntPtr       '  pointer to array of details_XX structs
    End Structure
    <StructLayout(LayoutKind.Sequential)> _
    Private Structure MIXERCONTROLDETAILS_UNSIGNED
        <FieldOffset(0)> Public dwValue As Integer        '  value of the control
    End Structure
    <StructLayout(LayoutKind.Sequential)> _
    Private Structure MIXERLINECONTROLS
        <FieldOffset(0)> Public cbStruct As Integer       '  size in Byte of MIXERLINECONTROLS
        <FieldOffset(4)> Public dwLineID As Integer       '  line id (from MIXERLINE.dwLineID)
        <FieldOffset(8)> Public dwControl As Integer      '  MIXER_GETLINECONTROLSF_ONEBYTYPE
        <FieldOffset(12)> Public cControls As Integer      '  count of controls pmxctrl points to
        <FieldOffset(16)> Public cbmxctrl As Integer       '  size in Byte of _one_ MIXERCONTROL
        <FieldOffset(20)> Public pamxctrl As IntPtr       '  pointer to first MIXERCONTROL array
    End Structure
    <StructLayout(LayoutKind.Sequential)> _
    Private Structure MIXERLINE
        <FieldOffset(0)> Public cbStruct As Integer                '  size of MIXERLINE structure
        <FieldOffset(4)> Public dwDestination As Integer          '  zero based destination index
        <FieldOffset(8)> Public dwSource As Integer               '  zero based source index (if source)
        <FieldOffset(12)> Public dwLineID As Integer               '  unique line id for mixer device
        <FieldOffset(16)> Public fdwLine As Integer                '  state/information about line
        <FieldOffset(20)> Public dwUser As Integer                 '  driver specific information
        <FieldOffset(24)> Public dwComponentType As Integer        '  component type line connects to
        <FieldOffset(28)> Public cChannels As Integer              '  number of channels line supports
        <FieldOffset(32)> Public cConnections As Integer           '  number of connections (possible)
        <FieldOffset(36)> Public cControls As Integer              '  number of controls at this line
        <FieldOffset(40), MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst:=MIXER_SHORT_NAME_CHARS)> Public szShortName As String  ' * MIXER_SHORT_NAME_CHARS
        <FieldOffset(56), MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst:=MIXER_LONG_NAME_CHARS)> Public szName As String ' * MIXER_LONG_NAME_CHARS
        <FieldOffset(120)> Public dwType As Integer
        <FieldOffset(124)> Public dwDeviceID As Integer
        <FieldOffset(128)> Public wMid As Integer
        <FieldOffset(132)> Public wPid As Integer
        <FieldOffset(136)> Public vDriverVersion As Integer
        <FieldOffset(168), MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst:=MAXPNAMELEN)> Public szPname As String ' * MAXPNAMELEN
    End Structure
#End Region
#Region "API Declarations"
    Private Declare Function mixerOpen Lib "winmm.dll" (ByRef phmx As Integer, <MarshalAs(UnmanagedType.U4)> ByVal uMxId As Integer, ByVal dwCallback As Integer, ByVal dwInstance As Integer, ByVal fdwOpen As Integer) As Integer
    Private Declare Function mixerGetLineInfo Lib "winmm.dll" Alias "mixerGetLineInfoA" (<MarshalAs(UnmanagedType.I4)> ByVal hmxobj As Integer, ByRef pmxl As MIXERLINE, ByVal fdwInfo As Integer) As Integer
    Private Declare Function mixerGetLineControls Lib "winmm.dll" Alias "mixerGetLineControlsA" (<MarshalAs(UnmanagedType.I4)> ByVal hmxobj As Integer, ByRef pmxlc As MIXERLINECONTROLS, ByVal fdwControls As Integer) As Integer
    Private Declare Function mixerSetControlDetails Lib "winmm.dll" (<MarshalAs(UnmanagedType.I4)> ByVal hmxobj As Integer, ByRef pmxcd As MIXERCONTROLDETAILS, ByVal fdwDetails As Integer) As Integer
#End Region

    Public Sub Mute(ByVal onoff As Boolean)
        Dim hmixer As Integer
        Dim volCtrl As New MIXERCONTROL
        Dim lngReturn As Integer
        Dim lngVolSetting As Integer
        lngReturn = mixerOpen(hmixer, 0, 0, 0, 0)
        If lngReturn <> 0 Then Exit Sub
        Call GetVolumeControl(hmixer, MIXERLINE_COMPONENTTYPE_DST_SPEAKERS, MIXERCONTROL_CONTROLTYPE_MUTE, volCtrl)
        If onoff Then
            lngVolSetting = 1
        Else
            lngVolSetting = 0
        End If
        SetVolumeControl(hmixer, volCtrl, lngVolSetting)
    End Sub
    Public Sub SetVolume(ByVal Level As Integer)
        Dim hmixer As Integer
        Dim volCtrl As New MIXERCONTROL
        Dim lngReturn As Integer
        Dim lngVolSetting As Integer
        lngReturn = mixerOpen(hmixer, 0, 0, 0, 0)
        If lngReturn <> 0 Then Exit Sub
        Call GetVolumeControl(hmixer, MIXERLINE_COMPONENTTYPE_DST_SPEAKERS, MIXERCONTROL_CONTROLTYPE_VOLUME, volCtrl)
        lngVolSetting = CType(volCtrl.lMaximum * (Level / 100), Integer)
        SetVolumeControl(hmixer, volCtrl, lngVolSetting)
    End Sub
    Private Function GetVolumeControl(ByVal hmixer As Integer, ByVal componentType As Integer, ByVal ctrlType As Integer, ByRef mxc As MIXERCONTROL) As Boolean
        Dim mxlc As New MIXERLINECONTROLS
        Dim mxl As New MIXERLINE
        Dim rc As Integer, pmem As IntPtr
        mxl.cbStruct = Marshal.SizeOf(mxl)
        mxl.dwComponentType = componentType
        rc = mixerGetLineInfo(hmixer, mxl, MIXER_GETLINEINFOF_COMPONENTTYPE)
        If (MMSYSERR_NOERROR = rc) Then
            mxlc.cbStruct = Marshal.SizeOf(mxlc)
            mxlc.dwLineID = mxl.dwLineID
            mxlc.dwControl = ctrlType
            mxlc.cControls = 1
            mxlc.cbmxctrl = Marshal.SizeOf(mxc)
            pmem = Marshal.AllocHGlobal(Marshal.SizeOf(mxc))
            mxlc.pamxctrl = pmem
            mxc.cbStruct = Marshal.SizeOf(mxc)
            rc = mixerGetLineControls(hmixer, mxlc, MIXER_GETLINECONTROLSF_ONEBYTYPE)

            If (MMSYSERR_NOERROR = rc) Then
                mxc = CType(Marshal.PtrToStructure(mxlc.pamxctrl, GetType(MIXERCONTROL)), MIXERCONTROL)
                Marshal.FreeHGlobal(pmem)
                Return True
            End If
            Marshal.FreeHGlobal(pmem)
            Return True
            Exit Function
        End If
        Return False
    End Function
    Private Function SetVolumeControl(ByVal hmixer As Integer, ByVal mxc As MIXERCONTROL, ByVal volume As Integer) As Boolean
        Dim mxcd As MIXERCONTROLDETAILS
        Dim vol As MIXERCONTROLDETAILS_UNSIGNED
        Dim rc As Integer
        Dim hptr As IntPtr
        mxcd.item = 0
        mxcd.dwControlID = mxc.dwControlID
        mxcd.cbStruct = Marshal.SizeOf(mxcd)
        mxcd.cbDetails = Marshal.SizeOf(vol)
        hptr = Marshal.AllocHGlobal(Marshal.SizeOf(vol))
        mxcd.paDetails = hptr
        mxcd.cChannels = 1
        vol.dwValue = volume
        Marshal.StructureToPtr(vol, hptr, False)
        rc = mixerSetControlDetails(hmixer, mxcd, 0)
        Marshal.FreeHGlobal(hptr)
        If (MMSYSERR_NOERROR = rc) Then
            Return True
        Else
            Return False
        End If
    End Function
End Class
Does anyone know how I can manipulate this class to also change the system speaker volume or is there another class I can use?

Basically I want to have a function that:
Private Sub speachBTN_Click(sender As Object, e As EventArgs) Handles speachBTN.Click
dim current volume
unmute if muted
set volume to max (65535 or 100%)

do same as above for application mixer
speaker computer voice
reset system volume back to the initial volume (dim current)
End Sub

I have the speaker voice and application volumes done, just need the system volume.