How would one go about making a music or sound file such as a .midi or .ogg to fade in or out.
I believe you can load the sound into a /sound object and then adjust its volume from there. So if you assign each client a music var and then assign a certain sound to that variable, you should be able to gradually increase or decrease the variable to create fade effects.

I haven't tried that, but it sounds good.
In response to Foomer
I believe you made a post on this a few months ago, and I replied with a snippet with it. Search for that, OP, I don't remember how good it was though.