To insert a Master Sound Track that you want to loop you
must enter the track in the first Slide in the show.
Select the first slide. Double click on it or From Edit
select Slide Properties.
Select the Sound tab as shown above.
Click on Include Slide Sound.
Click on Browse and select your sound file to be used as
the Master Sound Track.
Under Time, click on Repeat Continuously (as shown
above).
The Sound file you have selected will repeat until the end of the
show. If you want more then one sound file in your Master
Track then use a third party sound editor (e.g. Sound Recorder found
on every PC) to combine sound files into a single Master file.
The links below provide additional information on using sound.