Okay, seems we have a common device named now, that will help.
I've asked a playtester who had played on an iPad Air 2 to see if sound is a problem now with him. (It hadn't been before, so iOS version could be a factor.)
Will let you know results and maybe that will help track things down further.
Keep the reports coming! Some new tidbit of info would still help. Thanks, all!
UPDATE:
Playtester was actually on iOS 9 beta, but his iPad Air 2 played GK just fine, he said.
So we're still looking for more accounts of between iOS 8.3 and 9. But at least we know iPad Air 2 as a device should handle the game fine.