I don't know how to get a decent link from Amazon but "FiiO D3 Digital to Analog Audio Converter" works pretty well. $30.

There's a bug in some AppleTV where it has to be connected to a TV on boot or there's random noise from the digital audio out, but the easy solution is just keep it on (it only draws like 1 watt idle anyway). Hook up to a dedicated amplified speakers and you have wireless audio whenever.