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honestly, just use MIDI for now, until it gets to the point where MP3s can be loaded into quests and managing the loops and stuff, its really pointless to use them unless they loop from start to finish.
Wrong!
MP3 files won't be included into .qst files because that would blow up quest files beyond all proportion. Instead, just give players the MP3 files separately.
As to loops, yes it's far from perfect ... but until 2.5 is out don't expect too many new features.
Now to the actual problem: if you go into Zquest and select Etc./Play music, then your MP3 file ... does it play?
If not it's either your file or a ZC bug. If it does, select the file as dmap music and set the midi for that dmap to (none) for now. The idea is that you'll be able to set a midi and a mp3, and if the mp3 isn't found the midi will play, but I'm not sure that works yet.