Here's a lil sumptin.
This might be a case sensitivity issue. You have to have a folder named music, and a mention in the description.ext of a folder music, not Music or MuSiC or anything else but the exact name. If it's case sensitive that is.
Same goes for the file. Which has to be converted to .ogg in the right way so ofp recognizes it.
2ndly The class CfgMusic has to be the first class in the description.ext, before it goes the onLoadIntro="missionname"
onLoadMission="missionname" showCompass=1 showGPS=1...
stuff, and class CfgSounds follows, then class CfgRadio, then class Weapons after all sounds, then identities. If there is
};
};
before any class, then that class is not read and gives an error "input after end of file" if I remember.
that's all I can think of.