There are (at least) three ways one can use an Apple TV to play music:

(1) Airplay streaming from a computer, iPod, iPhone, etc. This works in pretty much the same way as airplay to an Airport Express, or some airplay-enabled speakers like the Zeppelin Air. The streaming is done close to real-time, with a player application (like iTunes, or the player software on an iPhone) controlling it. (Recent versions of OS X with the right hardware can do this from the operating system level, both for audio and video). Although it is potentially bit-perfect for 44.1 kHz lossless files with the Airport Express (but not for ATV, which then re-samples to 48kHz), in my experience it is not sufficiently robust, and it is plagued with annoying drop-outs.

(2) Buffered playback from the Apple TV itself. (This is what I called "zone player", probably erroniously). This relies on iTunes being open on a computer, so that it can act as a server for the music library (and video library )However, this is not real-time streaming, so it is more robust. Playback is controlled either from an iPod/iPhone/iPad running Apple's Remote.app, or using the Apple TV video interface via a monitor connected via HDMI to the ATV box, along with the IR remote (or iOS Remote.app).

(3) Stand-alone streaming from Apple: Movies, TV shows and music purchased from Apple (and music made available from iTunes match) can be streamed from Apple's servers instead of your own computer.

In my experience, (2) and (3) work better than (1). Also, you can play something different on each of your Apple TVs simultaneously.

In my case, I have 3 ATV units. One feeds my TV near my main system (and I have attached it to the main system as well via optical out, but I very seldom use it for audio on the main system, since I have a mac mini as well). Another feeds my first-generation (non-Air) Zeppelin via optical in my bedroom. It is "headless", so I need to control it via iOS Remote.app or stream to it via method (1). A third one is in my garage, which feeds video to a projector and audio to a cheap audio system, which I use when on my bike on a fluid trainer.

The main disadvantage compared to Sonos is that you still need an optical (or HDMI) DAC, amp and speakers. But that is also a feature that gives one more flexibility.