add missionbonus sound

recreated by Latvamage using milkytracker and the original
sample sent to the amiga audiochip and captured using winuae's
sample ripper.

see attached xm file.
remove mission bonus xm
add mission bonus sound to wavesounds
audio.c: make it possible to play pseudo empty tune

wave sounds are only played when music is playing as well, because
the sounds are mixed into the music stream.
having a separate logic and thread/lock state to deal with sounds
separately from music seemed like a lot of gratuitous effort,
so i just faked the functionality by playing a pseudo empty tune.

for some reason, when the empty tune starts playing, there's a short
unwanted noise.
music.h: add empty tune to music index
play missionbonus sound on mission end screen