uses
  MPlayer;

var
  mp : TMediaPlayer;

procedure TForm1.Button1Click(Sender: TObject);
begin
  with Sender as TButton do
    case Tag of
     0 :
       begin
         Tag := 1;
         mp := TMediaPlayer.CreateParented(Handle);
         mp.DeviceType := dtSequencer;
         mp.FileName := 'c:\winnt\media\Canyon.mid';
         mp.Wait:= True;
         mp.Open;
         mp.Play;
       end;
     1 :
       begin
         Tag := 0;
         mp.Wait := True;
         mp.Stop;
         mp.Free;
       end;
    end;
end;