It appears to have started occurring after I upgraded to Mavericks (OS X 10.9), though I can’t confirm this for sure. When I select an airplay output device from my Macbook Air nothing happens.
TLDR;
If you have this problem restart the music service for your mac with the following command:
sudo kill `ps -ax | grep 'coreaudiod' | grep 'sbin' |awk '{print $1}'`
THE DETAILS
If I hold the option key and click on the speaker icon in the menu bar, I see the Airplay devices available on my network and am able to select them:
However, after selecting a device the audio will continue to play from my Macbook Air, and if I re-open the menu under ‘Output Device:’ my ‘Internal Speakers’ are still selected.
Looking for a solution to this problem I came across this forum thread which suggested the following command, which I break down below:
##### Full Command #####
sudo kill `ps -ax | grep 'coreaudiod' | grep 'sbin' |awk '{print $1}'`
##### Breaking down the command #####
# Lists the running processes on your computer
ps -ax
# Finds the line for the core audio process in the process list
ps -ax | grep 'coreaudiod'
# Avoids the problem where the grep process also shows up in the ps results
ps -ax | grep 'coreaudiod' | grep 'sbin'
# Pulls out the Process ID for the core audio process
ps -ax | grep 'coreaudiod' | grep 'sbin' |awk '{print $1}'
# Uses the Process ID of the core audio process to kill it (it will auto-restart)
sudo kill `ps -ax | grep 'coreaudiod' | grep 'sbin' |awk '{print $1}'`
This command has been a life saver for me, though I’m still not sure why Apple’s core audio service would be so brittle to get into the bad state necessitating this command so frequently.