Q: AirPlay Mirroring

Is the volume low or muted on the computer?

Same problem here!  Just installed ML on my new MacBook Pro - airplay picture to HD TV is excellent but the sound is coing from the Laptop???  In system preferences (sound) my Appple TV shows up as an output source but I am unable to select it!!

Apple - any thoughts?

Same thing here. I hope it works, that is why I got a new computer in the first place!

Yes, I have the same problem. The Airplay sound option cannot be selected. Late 2011 MacBook Pro, just updated to Mountain Lion.

same problem on my brand new MBP-Retina. Haven't figured out a solution yet... anybody?

I just did a simple restart and now it's working... hope that works!

Happening on my new retina macbook with mountain lion too.  Rebooting seems to fix it - for a while.

This is happening to me as well but I found a solution.  Things I have noticed:

1.  Apple TV can't be selected (switches back to internal speakers)

2.  Airplay Display Mirroring continues to work

3.  Airplay from ITunes still works

4.  Seems to happen after the computer sleeps.

SOLUTION!

In the terminal, enter:

sudo dscacheutil -flushcache

sudo killall -HUP mDNSResponder

This isn't ideal, but it works. Flushing out the cache seems to remove whatever is causing the issue. 

Try rebooting your Mac and Apple TV. That is how i got mine to work. It is amazing what turning it off and on again can do. Reminds me of the good old Microsoft days. This product is pretty flakey!

This bug still occurs even with the latest updates installed of Mountain Lion. It is definitely a bug on Apple's part, a sign of a true lack of QA testing. I have spoken to AppleCare agents and they confirmed there is nothing wrong with what I am doing. Hopefully Apple will prioritize a fix for this in the next release cycle. AirPlay Mirroring is one of the major uses I have for my laptop now and I find this endlessly frustrating as I have to reboot almost daily to fix this problem.

I just experienced this for the first time on my 2011 Mac Air.  It has worked great up till now.

Weird.  I tried the dns cache flush but it did not work.

Will try reboot.

--- update

I rebooted Mac and Apple TV and streamed sound came back.  Thanks for the tip! 

This did happen right after an OSX update.  It did not prompt to reboot but I guess it was necessary.

I'm having the same issue, and it's persistent. Power-cycling everything resolves the issue temporarily, but it always comes back. Any other hints?