Hi PJ,
the only dumb questions are those that are not asked

In the Slide Show menu, click on Custom Animation...
That will bring up a task pane on the right of your screen titled Custom
Animation. After you have inserted a sound (Insert menu, Movies and sound,
Sound from file) and it is selected, you should see the sound file selected
in the custom animation task pane. Double click it in the taskpane (John
referred to the sound file you inserted as "media") which brings up a dialog
window. The effect tab will have the setting stop playing after x slides
John pointed out.
I hope this helps,
Patrick Schmid