I have an OS X box running iTunes on it. I play music with iTunes connected to an Apple TV. In iTunes I can select Apple TV as the output sound device. Am I able to do that with any other application? Apple TV does not show up as a seperate device in System Preferences. I'd like to browse and play YouTube videos in my computer and stream their sound to Apple TV.
You can’t by default, but Rogue Amoeba makes a $25 app called Airfoil that lets you send audio from any application to your AppleTV (or other remote speakers, like Airport Expresses).
I’ve used it for listening to Spotify via Airport Express, seems to work well.