uses MMSystem;
...
procedure TForm1.Button1Click(Sender: TObject);
begin
mciSendString('stop midi', '', 0, 0);
mciSendString('close midi', '', 0, 0);
mciSendString('open c:\prueba.mid type mpegvideo alias midi', '', 0, 0);
mciSendString('play midi repeat', nil, 0, 0);
end;