To play music or other audio across your entire presentation, set it up from the Format Audio tab.
Go to the Home tab and click Media > Audio Browser, then choose iTunes or GarageBand.
Select your audio file and drag it onto the first slide.
Tip: To preview a song, double-click it.
With the speaker icon selected, click the Format Audio tab and click Start > Play Across Slides.
Tip: If your presentation is longer than your audio, make the audio repeat by clicking Playback Options > Loop Until Stopped.
Tip: To hide the audio icon, select Hide Icon During Show.