The syntax is correct and it's worked for me like a charm. I'm fresh out of any solid ideas but here's a few thin ones.
In the description.ext try setting your music piece volume to +40db or something, so that the volume magnitude is larger to fiddle with.
If you're using a trigger's effects selection to start the music, take it away, and try playing the music with the
playmusic command from a script or a triggers on activation field or something.
Check the volume settings in your OFP options. Make sure they're not too low. (again to give some more room for the music volume to fade in).
When using the fademusic command, try a whole bunch of different values increasing/decresing them gradually, little by little.
I can't do anything better I'm afraid, but if a wise man comes along and solves the problem, or you manage to get it to work yourself, you could write that missing tutorial so that others won't have to suffer as you did