So you want to recreate Apple's AirPlay using Raspberry Pi ;)
For a ready solution supporting AirPlay playback, lifehacker has a solution: http://lifehacker.com/5978594/turn-a-raspberry-pi-into-an-airplay-receiver-for-streaming-music-in-your-living-room
but then you will need iTunes to serve the music stream.
If you want to build your own....
- For multi-room audio playback:
The keyword here is synchronisation, which can be achieved only if you have a central media server that sends out audio packets to multiple replay clients together with instructions on which precise moment in time to play each audio packet. I do not mean just a hard disk packet server, I mean the equivalent of an orchestra conductor to ensure that each client does exactly the same thing at the same time with everyone else. It would also need to send perfect clock (time) synchronisation signals, down to the millisecond. Any variations in time synch between clients would result in eerie echo-sounding audio artefacts.
Unfortunately, as sparkie says in his comment, there is no off-the-shelf solution that supports perfectly synchronised audio replay through the network. So the next option is to create a single media player.
- For single room audio playback:
This is much simpler. Here you do not need a "master conductor" as there is nothing requiring synchronisation. The audio can play back as soon as the packets arrive at the Pi. However, before you get into searching for suitable media software, have you considered who will start and stop the audio playback? Do you want a user interface on the speaker to control the music? Then it might be simpler to connect it to your tv with a wireless keyboard and mouse. OpenELEC is very nice: http://www.raspberrypi.org/archives/tag/media-centre and so is any XBMC implementation: http://lifehacker.com/5929913/build-a-xbmc-media-center-with-a-35-raspberry-pi
Or are you happy to use a full server in another room? Then you need to go to the other room to start and stop the music, select tracks, etc. Or use a phone or table maybe. Here is one suggestion: http://www.audiostream.com/content/raspberry-pi-half-baked-50-network-player and here is a discussion on a forum: http://forum.stmlabs.com/showthread.php?tid=4354
Also have a look at the RPi MPD (Raspberry Pi Music Player Daemon) http://elinux.org/Rpi_Music_Player_Daemon