A recent page on the AwkwardTV wiki (a great resource, by the way) has details on how you can stream any audio (not just iTunes) to your Apple TV. This is great because it's almost like a software version of the AirPort Express, which I kind of regret selling now because it was such a great little device for this kind of thing. I kind of missed being able to open iTunes and have it play automatically on the stereo in the living room.

Here's the instructions.
»wiki.awkwardtv.org/wiki/Stream_A···AppleTV

Thanks goes to Maraklov who documented this. I had never even heard of Soundfly before.

The first requisite is that you'll need a hacked Apple TV. There are many ways to accomplish that these days, and I'll get into that in a later post, but as long as you can remotely copy a file to the device (I like scp) then you're good.

Now install Soundfly, a utility designed to send audio from one computer to another using Soundflower.

The last thing you need to do, and it's literally this straight forward, is copy the Soundfly receiver app to the Apple TV and run it. You do this using the open command, which launches an OS X app from the command line. So, assuming you copied the receiver app to ~/Applications/:

open /mnt/Scratch/Users/frontrow/Applications/Soundfly\ receiver.app
 

Lastly on the wiki page are instructions for making the app start automatically when the Apple TV starts, done through the "correct" method of using LaunchServices. But I suppose you could also do it the quick and dirty way, by adding the command to /etc/rc.local.

Now just launch the Soundfly app on your computer and start playing audio, and like magic, the sound is streamed to the Apple TV. The app is basically invisible, so you won't see anything displayed on the TV screen, aside from the regular UI.

I played with it a little and it works great! Just another way to extend the functionality of your Apple TV, all thanks to the hacker community at AwkwardTV.