{by Serhiy Perevoznyk}

 uses
 MMSystem;


 function GetVolumeControl(aMixer: HMixer; componentType, ctrlType: Longint;
 var mxc: TMixerControl): Boolean;
 var
 mxl: TMixerLine;
 mxlc: TMixerLineControls;
 rc: Longint;
 begin
 Result := False;
 FillChar(mxl, SizeOf(TMixerLine), 0);
 mxl.cbStruct := SizeOf(TMixerLine);
 mxl.dwComponentType := componentType;
 {Obtain a line corresponding to the component type}
 rc := mixerGetLineInfo(aMixer, @mxl, MIXER_GETLINEINFOF_COMPONENTTYPE);
 if rc = MMSYSERR_NOERROR then
 begin
 with mxlc do
 begin
 cbStruct := SizeOf(TMixerLineControls);
 dwLineID := mxl.dwLineID;
 dwControlType := ctrlType;
 cControls := 1;
 cbmxctrl := SizeOf(TMixerLine);
 pamxctrl := @mxc;
 pamxctrl^.cbStruct := SizeOf(TMixerControl);
 end;
 mixerGetLineControls(aMixer, @mxlc, MIXER_GETLINECONTROLSF_ONEBYTYPE);
 rc := mixerGetLineControls(aMixer, @mxlc, MIXER_GETLINECONTROLSF_ONEBYTYPE);
 Result := rc = MMSYSERR_NOERROR;
 end;
 end;

 function SetVolumeControl(aMixer: HMixer; mxc: TMixerControl; volume: Longint): Boolean;
 var
 mxcd: TMixerControlDetails;
 vol: TMixerControlDetails_Unsigned;
 rc: MMRESULT;
 begin
 FillChar(mxcd, SizeOf(mxcd), 0);
 with mxcd do
 begin
 cbStruct := SizeOf(TMixerControlDetails);
 dwControlID := mxc.dwControlID;
 cbDetails := SizeOf(TMixerControlDetails_Unsigned);
 paDetails := @vol;
 cMultipleItems := 0;
 cChannels := 1;
 end;
 vol.dwValue := volume;
 rc := mixerSetControlDetails(aMixer, @mxcd, MIXER_SETCONTROLDETAILSF_VALUE);
 Result := rc = MMSYSERR_NOERROR;
 end;

 function InitMixer: HMixer;
 var
 Err: MMRESULT;
 begin
 Err := mixerOpen(@Result, 0, 0, 0, 0);
 if Err <> MMSYSERR_NOERROR then
 Result := 0;
 end;

 // Example: 


procedure SetMasterVolumeToZero;
 var
 MyMixerHandle: HMixer;
 MyVolCtrl: TMixerControl;
 begin
 MyMixerHandle := InitMixer;
 if MyMixerHandle <> 0 then
 try
 FillChar(MyVolCtrl, SizeOf(MyVolCtrl), 0);
 if GetVolumeControl(MyMixerHandle, MIXERLINE_COMPONENTTYPE_DST_SPEAKERS,
 MIXERCONTROL_CONTROLTYPE_VOLUME, MyVolCtrl) then
 begin
 {The last parameter (0) here is the volume level}
 if SetVolumeControl(MyMixerHandle, MyVolCtrl, 0) then
 ShowMessage('Volume should now be set to zero');
 end;
 finally
 mixerClose(MyMixerHandle);
 end;
 end;


 procedure TForm1.Button2Click(Sender: TObject);
 begin
 SetMasterVolumeToZero
 end;