Good luck to anyone working with .j2b! I had a go once - there's a header for them in the modplug player or something, but nothing more than that (so it won't actually play them). Some technical info for those who are interested: the files have a header containing checksums and file length, and then everything after that is gzipped. What you get after ungzipping is basically a RIFF file, but with some differences (chunks other than LIST contain
other chunks, which threw off RIFFpad). I think I got somewhere with the basic format, but gave up. Search the jcf and you should be able to find what I managed.

With Unreal Tournament, XMPlay plays the .umx files natively (.umx is a container format usually containing a s3m or similar). With XMPlay you can feed the output to somehting like LAME or oggenc.