Get Airplay Audio Working When it Refuses to Play Along

I've found recently (both at the office and at home) that sometimes Airplay decides it doesn't want to let you play audio to your Apple TV, or in my case Apple TV and/or Airport Extreme (mirroring seems to always work). This manifests itself as the receiver showing up in your output audio sources but you can't select it.

Turns out there is some weird bug and the quickest way to get things working again without restarting is to restart Core Audio:

sudo kill `ps -ax | grep 'coreaudiod' | grep 'sbin' | awk '{print $1}'`

That is, find the coreaudiod process, and pass it's process id (first argument) to kill.