Activating the Apple Thunderbolt Display speakers when Macbook Pro is docked
I've experienced this annoyance too. Rebooting can be very inconvenient when I have a lot of work open. Unplug/plugging the monitor doesn't seem to help. After reading the answer above, I started to log out, canceled it and for some reason the display audio option was restored. Woohoo! I'll repeat same procedure next time and see if this was just luck.