Wow this rabbit hole was deeper than expected... Got lha working in the process so was worth it (only libpokecubed source I found was in .lha). As the first source of oldplayer I found was 0.99 used that. Got it working, though not ready yet (some libs in the repos do not work, some are missing, keys needs adjusting). Working so far:

1 out of 2 .sid files
.it (1of1)
.med (as above)
.mod (2of2)
.s3m (1of1)
.xm (1of1)

Not working:
.ahx (crashes)
.fst (not listed in directory view)
.ogg (as above)
.vgm (unplayable format msg displayed)

.mp3 plays at about 4x speed

No touchscreen support (gp2x doesn't have one I guess)
Does open .zip files (.7z do not show up in filelist view, so unlikely .99 supports that)

Will try to find 1.10 (or whichever is latest) but will need static lib (libmodplug from repos doesn't work, had to statically include in start dir and LD_LIBRARY_PATH trick to get it working)...