Try this (found it on deja)
1. Insert your sound from file (midi, WAV, etc.). When asked, respond yes to "Play automatically?".
2. With the speaker icon selected, go to Slide Show > Custom Animation and click on the Multimedia Settings tab.
3. Under "While Playing" select "Continue Slide Show"
4. Under "Stop Playing" set a very large value in the number of slides (perhaps 3 times the number of slides in your presentation).
5. Click on "More Options" and select "Loop until stopped".