Music files...I got an error for .it files.

[Xion]

Strange...ICQ me for the real file I had to convert to xm to get it to work with my level.

[Haze]

AFAIK the music engine used in JJ2 has very limited Impulse Tracker (.IT) support. Several effects are missing and there's no proper support for instruments.
Furthermore when you're using a program such as ModPlug Tracker to make your .IT you must remember that .IT-files written in ModPlug may contain some non-standard features.

I'll probably post a detailed doc on this one of these days.