uses
 MMSystem;

function GetMasterMute(
 Mixer: hMixerObj;
 var Control: TMixerControl): MMResult;
// Returns True on success
var
 Line: TMixerLine;
 Controls: TMixerLineControls;
begin
 ZeroMemory(@Line, SizeOf(Line));
 Line.cbStruct := SizeOf(Line);
 Line.dwComponentType := MIXERLINE_COMPONENTTYPE_DST_SPEAKERS;
 Result := mixerGetLineInfo(Mixer, @Line,
 MIXER_GETLINEINFOF_COMPONENTTYPE);
 if Result = MMSYSERR_NOERROR then
 begin
 ZeroMemory(@Controls, SizeOf(Controls));
 Controls.cbStruct := SizeOf(Controls);
 Controls.dwLineID := Line.dwLineID;
 Controls.cControls := 1;
 Controls.dwControlType := MIXERCONTROL_CONTROLTYPE_MUTE;
 Controls.cbmxctrl := SizeOf(Control);
 Controls.pamxctrl := @Control;
 Result := mixerGetLineControls(Mixer, @Controls,
 MIXER_GETLINECONTROLSF_ONEBYTYPE);
 end;
end;

procedure SetMasterMuteValue(
 Mixer: hMixerObj;
 Value: Boolean);
var
 MasterMute: TMixerControl;
 Details: TMixerControlDetails;
 BoolDetails: TMixerControlDetailsBoolean;
 Code: MMResult;
begin
 Code := GetMasterMute(0, MasterMute);
 if Code = MMSYSERR_NOERROR then
 begin
 with Details do
 begin
 cbStruct := SizeOf(Details);
 dwControlID := MasterMute.dwControlID;
 cChannels := 1;
 cMultipleItems := 0;
 cbDetails := SizeOf(BoolDetails);
 paDetails := @BoolDetails;
 end;
 LongBool(BoolDetails.fValue) := Value;
 Code := mixerSetControlDetails(0, @Details,
 MIXER_SETCONTROLDETAILSF_VALUE);
 end;
 if Code <> MMSYSERR_NOERROR then
 raise Exception.CreateFmt('SetMasterMuteValue failure, ' +
 'multimedia system error #%d', [Code]);
end;

// Example:

procedure TForm1.Button1Click(Sender: TObject);
begin
 SetMasterMuteValue(0, CheckBox1.Checked); // Mixer device #0 mute on/off
end;