Background music track across all slides in presenter 10?

Short answer, no. Presenter doesn't have background audio functionality. You could use Captivate, as it does that that function.

Long answer, yes, but you need an audio editing program that would allow you to place a second audio track with your narration and then publish it out as a single audio file (or audio file per slide) and then insert into Presenter. This would allow there to be background music, but it wouldn't be independent of the narration.