1.First I found the longest mp3 music on my hard drive 2:48 minutes
2.On my first slide at the "sound" drop down window
3. loaded the mp3 music to Impress and selected it
4. Checked "loop until next sound"
5. On the following slides, selected <no sound>
6."Advance Slide Auto"
7. Start Slide Show
The thing is to have mp3 that is as long as your slide show that way the music will not have to loop that often.if at all.

Hope this helps