Автор: http://www.swissdelphicenter.ch

 uses
   MMSystem;

 procedure SetVolume(const volL, volR: Word);
 var
   hWO: HWAVEOUT;
   waveF: TWAVEFORMATEX;
   vol: DWORD;
 begin
   // init TWAVEFORMATEX 
  FillChar(waveF, SizeOf(waveF), 0);
   // open WaveMapper = std output of playsound 
  waveOutOpen(@hWO, WAVE_MAPPER, @waveF, 0, 0, 0);
   vol := volL + volR shl 16;
   // set volume 
  waveOutSetVolume(hWO, vol);
   waveOutClose(hWO);
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   SetVolume(14000, 14000);
 end;