I would suggest this AVFoundation framework for a start. You can set pan, volume, and also play from time, etc.
On my first app I used it with touches ended event. Every object had its own sound, and I played it when the user touched the object.
Also there is the OpenAL framework, but I am still a newbie in that...
Maybe my question was not that precise. I like to use AudioGraphs which works pretty well for me. AVFoundation can be used to trigger single sounds but thats not what I mean. Think of a soundtrack that dynamically changes its mood depending on gamestate.
Right now I'm looking for a MusicCompositing tool with an integrated script language. (Freeware would be best)