Within the program designer, a background audio zone can be added and synced to content while the programming loop plays.  To add an audio zone, click the zone icon with the blue music note in the tool bar.

Content in your programming is synced with the audio files in the audio zone.  After you create the audio zone, drag and drop the mp3 files.  Then, sync the zone (seen here as Zone A) to the audio zone.  Do this by clicking and dragging the blue M puzzle piece over to the orange puzzle piece in the audio zone's heading.  Once the sync is complete the audio zone's puzzle piece will turn to a green S.  

Next, choose which content files will be synced up with mp3s in the audio zone.  Drag a tracks puzzle piece over to the file's puzzle piece as seen below.  When finished, scroll your mouse over a synced piece of audio content to see more information in the 'My Master Track' pop up window.

Note: Be aware of the length of a programming file and the length of an audio mp3.  Use silent track mp3s to prevent audio tracks playing over each other.