I've voided this out on other sound interfaces than SDL Mixer ones because I'm both not sure whether they need it, and not sure how to make them work with it if they do.
Until we use something besides Native MIDI to play
back MIDI music, MIDI volume changing is disabled
since it causes way too much of a damn headache.
(It's not even our fault, it's fucking MS.)
Use whatever names you want for your music. So long as you prefix the lumps with O_ or D_, it doesn't matter anymore.
DISCLAIMER: Linedef type 413 (change music) and Lua scripting is not tested.
(cherry picked from commit 025ca413a2)
# Conflicts:
# src/p_user.c
