Автор: http://www.swissdelphicenter.ch

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;