I've been running Chuck on a MacPro with a Cinema Display, and I notice that the audio device switching isn't handled well by either the hardware/OS, or other audio apps. Often I have to diddle around with the Sound Preferences output to get Amadeus or Reaper to play to my headphones, or the display audio, instead of the internal speaker. So perhaps this has something to do with it.
I can't promise this will help, but I'm a fan of running Rogue Amoeba's free Sound Source app on OS X which makes selecting the machine's audio output really quick and simple:
(bottom of page)
It overrides the Mac's automatic device switching so just may be a fix. I don't know if it works on Lion - it's a bit old - but it functions great on Snow Leopard.
Anthony.