I haven't solved this problem yet. I have an Airport Express that ambar gave that runs my primary speakers, but I keep needing to build a netbsd version of raop-play to drive it off of midgard (or throw in the towel and finally migrate to linux). And I haven't tried to solve multi-room speaker systems yet (the Airport solution won't do that, because they bind each set of speakers to a specific source, and there's no way to maintain time synchronization between Airport Expresses.