You probably don't want to have Sound Effects and Music on the same source. You have far less control over either of them that way. I would still keep them separate, but just make both Output to the same mixer group instead.
You might need Source version, which I assume you have if you've been able to put sound and music onto the same audio source. I've tried to find scripting references for using Audio Mixers, but I'm not having much luck. It is on the cards for one of my projects, so I'll return here with my findings once I get up to it.