Why not *MusicChannel? Maybe it doesn't look that good alone, but if it takes the channel number as first parameter, it wold be something like:
Game.StopMusicChannel(3);
Very straightforward and clear when it comes to coding IMHO ... Or maybe an optional parameter: if it's empty, then it works as the old method, and if there's some number, it uses that channel. It's not like with the fades methods because for PlayMusic there are currently only 2 parameters, and I think than a third optional one will not be that annoying. And moreover, for the rest of music functions, there's no parameter at all right now... (except for SetMusicPosition)
And about fades... you're right, there's async taken, I didn't know, so... maybe Fade*Global or something like that? Something short that shows that the process is independent from the rest of the main game loop... Fade*Daemon? But it's not exactly that... I'm out of ideas right now, but if something worthy comes to my mind, I'll post it.