Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

uses
   MPlayer, MMSystem;

 const
   MCI_SETAUDIO = $0873;
   MCI_DGV_SETAUDIO_VOLUME = $4002;
   MCI_DGV_SETAUDIO_ITEM = $00800000;
   MCI_DGV_SETAUDIO_VALUE = $01000000;
   MCI_DGV_STATUS_VOLUME = $4019;

 type
   MCI_DGV_SETAUDIO_PARMS = record
     dwCallback: DWORD;
     dwItem: DWORD;
     dwValue: DWORD;
     dwOver: DWORD;
     lpstrAlgorithm: PChar;
     lpstrQuality: PChar;
   end;

 type
   MCI_STATUS_PARMS = record
     dwCallback: DWORD;
     dwReturn: DWORD;
     dwItem: DWORD;
     dwTrack: DWORD;
   end;

 procedure SetMPVolume(MP: TMediaPlayer; Volume: Integer);
   { Volume: 0 - 1000 }
 var
   p: MCI_DGV_SETAUDIO_PARMS;
 begin
   { Volume: 0 - 1000 }
   p.dwCallback := 0;
   p.dwItem := MCI_DGV_SETAUDIO_VOLUME;
   p.dwValue := Volume;
   p.dwOver := 0;
   p.lpstrAlgorithm := nil;
   p.lpstrQuality := nil;
   mciSendCommand(MP.DeviceID, MCI_SETAUDIO,
     MCI_DGV_SETAUDIO_VALUE or MCI_DGV_SETAUDIO_ITEM, Cardinal(@p));
 end;

 function GetMPVolume(MP: TMediaPlayer): Integer;
 var
    p: MCI_STATUS_PARMS;
 begin
   p.dwCallback := 0;
   p.dwItem := MCI_DGV_STATUS_VOLUME;
   mciSendCommand(MP.DeviceID, MCI_STATUS, MCI_STATUS_ITEM, Cardinal(@p));
   Result := p.dwReturn;
   { Volume: 0 - 1000 }
 end;

 // Example, Beispiel: 

procedure TForm1.Button1Click(Sender: TObject);
 begin
   SetMPVolume(MediaPlayer1, 500);
 end;