To import a background audio:
Click the Import Background Audio button to launch the Import Background Audio window.
Navigate to the desired audio in 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.
Click Open to import the background audio to the presentation 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.