O lookie, I'm taking an interest in this! ^_^
HappyFeet probably is trying to insert an s3m, xm, or it file. I tried to, because theoretically it should be able to play those along with MOD, as they're all supported by BASS.DLL. And when I included them, I got that. Just included .mod, and it made the style, but here's where my problem crops up...
I open my style in Wledit2x, go to set the music, pick music #1, which was replaced, and attempt to play it. I get the "cannot play this music" error. But, I thought it might just be an editor problem, so I selected music 1 anyway and made a level, then went into the hacked version of the game. The level music was silent-- nothing played. I can confirm, the files I used were valid MODs. The style in question simply takes the graphics from Orig-Fire.sty but adds MOD music. And yes, I have most stuff except the GFX and Objects referenced to Default.sty. If you want, I'll post the STT file.