I don't have an AppleTV (yet), but I do have a Mac Mini hooked up to some decent speakers. Is there something I can run on it, so that it acts like an Airplay audio receiver?
(Video support isn't important to me.)
From https://code.google.com/p/open-airplay/ (note they also support audio):
And as Christian says, AirFoil Speakers should also do the trick ($25 / 10 minute trial).
I personally had problems with AirServer (kept crapping out) and Reflector only gives you a 10 minute trial, which isn't enough.
Xbmc supports being an airplay receiver and runs on macs.