Can't play mp3 directly using DirectX.
You can use mci - here's how I play the mp3 in my demos
mciSendString("open \"Rezistah - Cauldron.mp3\" alias mympg", NULL,0,NULL);
mciSendString("play mympg repeat", NULL,0,NULL);
If you go to MSDN help for mciSendString and then 'play', there is a 'from' command which might help you. And a 'stop'.
Jim