From experience it's quite a job to make sfx/music share 4 channels, to make it not cut off notes. For the gameplay (in action games), you absolutely need to start the sfx exactly when they're triggered and not skip a single one, or the player will wonder if he jumped or picked up something etc. At the same time with this "absolute priority", you don't want the music to cut off randomly
You can have a sfx/music toggle, this is what I always like best in games. Because after a while you've heard the music a few times before...
A simple solution is to have 3-channel music of which 1 is a shared, low-priority-music channel (melody). In this channel, low-priority-sfx (shots, environment) win over music. The 4th channel is reserved for normal-priority-sfx only, so the important sounds like rewards and feedback sounds like thuds are always heard.
Since it is per-channel priority, you don't need a priority scheme, really. Important sounds are simply played in 4th chan, all others in chan 3 (simply play them there, it's ok if music in this chan is cut off), and don't touch chan 1-2.