In Powerpoint, how do I get the sound I imported to play across the entire slide show?

I have a Mac computer so my powerpoint works a little differently. I finally figured out how to go to the Clip Art online place and download some free music. I imported this one clip art audio clip I liked and I made it play automatically when the slide show starts. But when I play the slide show, it only plays for that ONE slide I put it on. How can I make it so the sound clip will play over all of the clips.