A while back my MBP (any MBP, actually – I tried 3) stopped showing Display Audio as an option for audio output in System Preferences, when plugged in to my Thunderbolt display.
Finally stumbled across the solution this morning in Apple Discussions, and it involves resetting the SMC:
- Shut Down your laptop
- Unplug Display(both power and thunderbolt connection) for 15 sec or so
- Press & Release Shift+Control+Option+Power
- Wait for 15 sec or so
- Plug back display and power
- Power