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Insert your sound using Insert > Movies and Sounds > Sound from File. When prompted, reply yes to "Do you want your sound to play automatically in the slide show?"
Right click on the sound icon and select Custom Animation. In the task pane your sound file will be listed under Custom Animation. Click on the down arrow next to it and select Effect Options > Effect. Under "Stop Playing" set it to "After 999 slides". Click on the Timing tab and select "Repeat" Until End of Slide.