Would this help?

(snippets from this thread over at the Un4seen forums)

"In fact J2B support was worked on for ModPlug's engine, however Olivier either gave up because of the encryption or just plainly couldn't care. All that remains is a blank header file which is STILL distributed with libmodplug.
So currently there's no plugin capable of playing the J2B-format."

"The only real difference probably is the "encryption", which IIRC correctly was merely a custom-header and some compression thrown over the file."

"J2B format contains a custom module format based on the RIFF chunked format, compressed using the deflate algorithm."

(and the best bit...)

"Maybe someone still wants to figure out the RIFF AM module format. Here's some basic info on the J2B format...

DWORD id; // "MUSE"
DWORD magic; // DE AD BE AF (huhuhuh, nice spelling)
DWORD filelen; // length of the entire file
DWORD csum; // checksum? doesn't appear to be CRC32 of decompressed data, maybe CRC32 of compressed data?
DWORD clen; // length of the compressed data
DWORD ulen; // length of the decompressed data
BYTE data[]; // clen bytes worth of compressed data; use zlib uncompress() to extract ulen bytes

Once you have that decompressed... good luck reverse engineering the underlying format. It might not be so hard, considering the existence of mod2j2b."

So now we know a bit more - it's compressed, and non-standard inside.