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(15535, 15535);
end;