Here's what I did:
1) File > New > Presentation > Create (new presentation with all default settings)
2) Panel at right: Slide Transition > Modify Transition > Sound = "apert" (fourth in the drop down list)
You should hear a short (~2s) sound at this point, as the effect is "previewed". If you don't hear it, try the "Play" button at the bottom of the Slide Transition panel. If that doesn't work, then OOo is not able to play sounds and you have to fix that before proceeding.
3) Slide Transition > Modify Transition > Sound = "Other sound" > (select your long music file).
You may or may not hear the preview for this, it depends on whether your music file has a quiet leader at the beginning. If the audio starts right away, you should hear it. If the audio starts after 1-2s of silence, you won't hear anything.
4) Click "Slide show" (bottom of the Slide Transition panel) or F5.
You should hear the music start. It will continue to play even after you advance to the next slide (which is the black "End of show" screen. Advance again to end the show and the music should stop.
I tested this with OOo 2.3 on Fedora Linux, using a .wav audio file. YMMV depending on what audio format your music is stored in; almost anything will play .wav files, IME.
I was not able to get the sound to repeat, except by restarting the show.
AOO 4 • Linux • Fedora 17