Shawn, I have some familiarity with Onkyo units, although not specifically the 5508. You really haven't given us any details other than "all hooked up" and "picture but no sound". Have you studied the manual carefully, especially what I'm sure must be several suggestions in the trouble-shooting section re audio? We don't even know at this point if you're talking about a DVD player or cable box for the source material and guessing has too many possibilities.

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Enjoy the music, not the equipment.