Well, since new levels require new music, I also discovered that JJ2+ supports MPTM modules.

MPTM is the OpenMPT Module format. It can be used in several games. I can also convert MPTM to MP3 or OGG, since OPL samples in MPTM modules in JJ2+ won't work.

If you want to use MPTM modules, do it at your own risk.