Add support MP3 as BGM

[Hykem]

Hmm, odd...
It looks like it can't find the MP3 folder. Could you please check if your friend did the following:
- Open ms0 folder;
- Open PSP folder;
- Create a new folder called MUSIC (must be capitalized, "MUSIC" and not "Music");
- Place the MP3 files inside this folder.

Thanks!