To import a background audio:
Click the Import Background Audio button to launch the Import Background Audio
window.
Select the Looped playback checkbox to enable looping of the background sound if necessary. The selected audio file will play through the presentation in a continuous loop.
or
click Cancel to exit without importing the file.The selected audio file will be added to the timeline. You can change looping settings right on the timeline by clicking the Looped playback button.