Transition sounds play across slides by default but AFAIK they cannot be made to loop. The only possibility is to use a free sound editor like Audacity and add some sound to make the original longer.
To recover the sound if you don't have the original try saving Type = Html Web Page. You should get a folder with all dependant files (including the wav file)
You could then insert it "normally" and set it to stop after 999 slides and repeat.
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