Wow, this is really subversive marketing on Apple's part. I've been wanting to get audio out of my flatscrren for a long time--too cheap to buy a proper home audio setup (plus I'm only interested in 2.1, until I get bored with that).
There was no language on any of the AirPlay-branded stuff for iOS 6 or AppleTV 5.1 that would have kept me from thinking this was possible--to get ALL audio from the Apple TV to another AirPlay device.
I lurked on Craigslist for days after realizing my version 1 AirportExpress wouldn't support this function. I found a new Base Station mini, as it were, only to find that it wouldn't play audio from video either (however, I did get it still sealed for $85). Which led me here. Why the heck would I want to stream music from my iTunes library from Apple TV to an AirPort Express, which is also connected to my Macs? I know, I know, Match. It's not worth making that a marketing point and making people angry when they realize what this feature actually is.
Steve would have never allowed this. This is Bozo-level stuff.
UPDATE: As I was typing this, I got suspicious. I plugged my gen 1 Airport Express back in and lo and behold it works fine for the music-only streaming function of AppleTV 5.1, which is counter to info I found elsewhere saying the gen 1 was a no go. Bozo-level! I bought this 2012 Airport Express for NOTHING.