Ну не знаю как с сенд стринг, а вот этот пример с миксером работает:

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Const MMSYSERR_NOERROR = 0

Const MAXPNAMELEN = 32

Const MIXER_LONG_NAME_CHARS = 64

Const MIXER_SHORT_NAME_CHARS = 16

Const MIXER_GETLINEINFOF_COMPONENTTYPE = &H3&

Const MIXER_GETLINECONTROLSF_ONEBYTYPE = &H2&

Const MIXER_SETCONTROLDETAILSF_VALUE = &H0&

Const MIXERLINE_COMPONENTTYPE_DST_FIRST = &H0&

Const MIXERLINE_COMPONENTTYPE_DST_SPEAKERS = &H4

Const MIXERCONTROL_CONTROLTYPE_VOLUME = &H50030001

Private Declare Function mixerOpen Lib "winmm.dll" (phmx As Long, _

ByVal uMxId As Long, ByVal dwCallback As Long, ByVal dwInstance As Long, _

ByVal fdwOpen As Long) As Long

Private Declare Function mixerGetLineInfo Lib "winmm.dll" Alias _

"mixerGetLineInfoA" (ByVal hmxobj As Long, pmxl As MIXERLINE, _

ByVal fdwInfo As Long) As Long

Private Declare Function mixerGetLineControls Lib "winmm.dll" Alias _

"mixerGetLineControlsA" (ByVal hmxobj As Long, pmxlc As MIXERLINECONTROLS, _

ByVal fdwControls As Long) As Long

Private Declare Function mixerSetControlDetails Lib "winmm.dll" (ByVal hmxobj _

As Long, pmxcd As MIXERCONTROLDETAILS, ByVal fdwDetails As Long) As Long

Private Declare Function mixerClose Lib "winmm.dll" (ByVal hmx As Long) As Long

Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" _

(Destination As Any, Source As Any, ByVal Length As Long)

Private Declare Function GlobalAlloc Lib "kernel32" (ByVal wFlags As Long, _

ByVal dwBytes As Long) As Long

Private Declare Function GlobalLock Lib "kernel32" (ByVal hmem As Long) As Long

Private Declare Function GlobalFree Lib "kernel32" (ByVal hmem As Long) As Long

Private Type MIXERCONTROL

cbStruct As Long

dwControlID As Long

dwControlType As Long

fdwControl As Long

cMultipleItems As Long

szShortName As String * MIXER_SHORT_NAME_CHARS

szName As String * MIXER_LONG_NAME_CHARS

lMinimum As Long

lMaximum As Long

reserved(10) As Long

End Type

Private Type MIXERCONTROLDETAILS

cbStruct As Long

dwControlID As Long

cChannels As Long

item As Long

cbDetails As Long

paDetails As Long

End Type

Private Type MIXERCONTROLDETAILS_UNSIGNED

dwValue As Long

End Type

Private Type MIXERLINE

cbStruct As Long

dwDestination As Long

dwSource As Long

dwLineID As Long

fdwLine As Long

dwUser As Long

dwComponentType As Long

cChannels As Long

cConnections As Long

cControls As Long

szShortName As String * MIXER_SHORT_NAME_CHARS

szName As String * MIXER_LONG_NAME_CHARS

dwType As Long

dwDeviceID As Long

wMid As Integer

wPid As Integer

vDriverVersion As Long

szPname As String * MAXPNAMELEN

End Type

Private Type MIXERLINECONTROLS

cbStruct As Long

dwLineID As Long

dwControl As Long

cControls As Long

cbmxctrl As Long

pamxctrl As Long

End Type

' Set the master volume level.

'

' VolumeLevel is the level value in percentage (0 = min, 100 = max)

' Returns True if successful

Function SetVolume(VolumeLevel As Long) As Boolean

Dim hmx As Long

Dim uMixerLine As MIXERLINE

Dim uMixerControl As MIXERCONTROL

Dim uMixerLineControls As MIXERLINECONTROLS

Dim uDetails As MIXERCONTROLDETAILS

Dim uUnsigned As MIXERCONTROLDETAILS_UNSIGNED

Dim RetValue As Long

Dim hmem As Long

' VolumeLevel value must be between 0 and 100

If VolumeLevel < 0 Or VolumeLevel > 100 Then GoTo error

' Open the mixer

RetValue = mixerOpen(hmx, 0, 0, 0, 0)

If RetValue <> MMSYSERR_NOERROR Then GoTo error

' Initialize MIXERLINE structure and call mixerGetLineInfo

uMixerLine.cbStruct = Len(uMixerLine)

uMixerLine.dwComponentType = MIXERLINE_COMPONENTTYPE_DST_SPEAKERS

RetValue = mixerGetLineInfo(hmx, uMixerLine, _

MIXER_GETLINEINFOF_COMPONENTTYPE)

If RetValue <> MMSYSERR_NOERROR Then GoTo error

' Initialize MIXERLINECONTROLS strucure and

' call mixerGetLineControls

uMixerLineControls.cbStruct = Len(uMixerLineControls)

uMixerLineControls.dwLineID = uMixerLine.dwLineID

uMixerLineControls.dwControl = MIXERCONTROL_CONTROLTYPE_VOLUME

uMixerLineControls.cControls = 1

uMixerLineControls.cbmxctrl = Len(uMixerControl)

' Allocate a buffer to receive the properties of the master volume control

' and put his address into uMixerLineControls.pamxctrl

hmem = GlobalAlloc(&H40, Len(uMixerControl))

uMixerLineControls.pamxctrl = GlobalLock(hmem)

uMixerControl.cbStruct = Len(uMixerControl)

RetValue = mixerGetLineControls(hmx, uMixerLineControls, _

MIXER_GETLINECONTROLSF_ONEBYTYPE)

If RetValue <> MMSYSERR_NOERROR Then GoTo error

' Copy data buffer into the uMixerControl structure

CopyMemory uMixerControl, ByVal uMixerLineControls.pamxctrl, _

Len(uMixerControl)

GlobalFree hmem

hmem = 0

uDetails.item = 0

uDetails.dwControlID = uMixerControl.dwControlID

uDetails.cbStruct = Len(uDetails)

uDetails.cbDetails = Len(uUnsigned)

' Allocate a buffer in which properties for the volume control are set

' and put his address into uDetails.paDetails

hmem = GlobalAlloc(&H40, Len(uUnsigned))

uDetails.paDetails = GlobalLock(hmem)

uDetails.cChannels = 1

uUnsigned.dwValue = CLng((VolumeLevel * uMixerControl.lMaximum) / 100)

CopyMemory ByVal uDetails.paDetails, uUnsigned, Len(uUnsigned)

' Set new volume level

RetValue = mixerSetControlDetails(hmx, uDetails, _

MIXER_SETCONTROLDETAILSF_VALUE)

GlobalFree hmem

hmem = 0

If RetValue <> MMSYSERR_NOERROR Then GoTo error

mixerClose hmx

' signal success

SetVolume = True

Exit Function

error:

' An error occurred

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