I had the exact same problem, here's how mine went:
When I first got it with all stock drivers it would play DVD's fine. I installed the Omega 4.7 equiv drivers and it wouldn't play DVD's anymore, they would just sit in the drive and spin. I backed the drivers down to the Omega 4.5's (not for this reason, I was just testing gameplay with different drivers) and the DVD's started playing again.
I know there are playback issues with 4.6 and up but I don't think they are related to getting the DVD to play. This makes no sense to me but you might want to try a different driver if you are using 4.6 or 4.7. Who knows, it could have been one of the reboots that fixed it. Rebooting cures a lot of things.