Keynote doesn't have a timeline editor. So if you want to sync transitions to the soundtrack:
- automate all of the transitions so the whole show can play through from start to finish
- export the whole show to a Quicktime movie file
- import that movie file into FCP X and use it's editing tools and timeline to drop in the soundtrack and then sync up the transitions exactly where you want them
FCP X may still have a free trial. If you don't have FCP X or it doesn't have a free trial and you don't want to buy it, other video editor software with a timeline could work as well. The key is some kind of timeline-based editor.
Else, setting up the music to play all the way through and then trying to time transitions to automatically advance and delay to hit soundtrack cues will likely drive you to madness. The way I suggest above will make it so you can get transitions to occur right when you want them to occur.
One other way is the way described here:
http://macintoshhowto.com/keynote/how-to-time-a-keynote-presentation-to-a-music-soundtrack.html. It will depend on you (manually) timing the transitions to align with the soundtrack all the way through. The FCP X (or other video editor) is THE best way IMO but this option doesn't require you to buy or learn any new software.
If it's only narration in the soundtrack, you can chop the soundtrack up into slide segments and then auto-play each segment when you transition to each new slide. But again, it's a bear to auto-time transitions so you might try the manual option at that link for each slide and then stitch all the individual slide videos together.
There are other tools out there (on the Windows side) that do what you want to do well. Articulate Presenter is one that does that well (but the FCP X option will cost much less).
