Click on the audio image
Select Slide Show > Custom Animation
In the Task Pane click on the audio animation drop down menu. Select Effect.
In the Play Sound dialog box select After in the Stop Playing section. Type
in the number of slide you want the sound to play for.
For more info go here:
"Play sounds across multiple slides (A WAV runs through it)"
http://www.rdpslides.com/pptfaq/FAQ00047.htm
Please let me know if this works for you.