Thanks XenonMan.

Ok...I had a response in progress but the problem seems to have resolved itself. I had connected the AppleTV's audio temporarily with an analog stereo patch cord but I switched the 975 back to use HDMI3 for audio (HDMI already being used for appletv video). While I was scratching my head my fiance (who to her credit is pretty tech savvy) toggled the 'surr mode' button on the front panel of the 975 and voila! Sound! ?!?! No idea why...maybe it needed a little burp? I had toggled surround modes using the mini remote but nothing was working before. I've since gone back to all HDMI inputs and audio is working!
I never love it when I don't understand why something starts working...but for now I'll take it. Fingers crossed it stays that way. I'll update if something changes.

Thanks all!

Adam