[NPIdea]

Como me he encontrado el mismo problema, pues doy la solución que me ha fucnionado, tal como lo he encontrado en una página.

Código Delphi [-]
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
  
   Mediaplayer.Filename:='Loquesea.avi';
  Mediaplayer.Open;
SetMPVolume(MediaPlayer1, 500);  // de 0 a 1000
  MediaPlayer.Play;
end;