I have a hard time believing you installed plus properly..or something just went horribly wrong. Did you let all the plus files replace the existing ones when you installed plus? You didn't just cherry-pick certain files in plus.zip? You have to take out all the files from the zip, then put into jj2, not put the zip in the jj2 folder (and no, you do not make a folder for it either). Another guess is you just put bass.dll in, but not much else.

Open plus from your desktop (not jcs). And check at the bottom left corner of the menu. If it just says "1.24" or "1.23" then it's not working. If it says "JJ2 Beta v4.3 (10-30-2013)" then plus should be up. If you tried to patch a version (god knows how) other than 1.23 or 1.24, that may explain why. Plus also gives a different intro image (splash) screen.

I also believe plus dropped support for WAV (or Blur just failed at doing that). Also, the music file may be so big, that jj2 has trouble supporting it. Or it's just bass.dll..I've had an IT file play like that.

I don't ever remember JJ2 getting messed up colours, unless I used 8-bit mode on fullscreen. Also, when you open from JCS, forget putting in the MP3 for now, exit from the level, and check the menu for the same things as the above paragraph.

Also, in single player, if you have no background put in, you get sprite trails like that.