This would be an easy setup. I've done similar projects with various hardware hacked-up to act as players (PSPs, Windows and PalmOS PDAs) but never found the ideal solution - and plan to use a RPi for just this application
Here's what I would (will) do:
The RPi will be effectively a massively over-powered mp3 stream player. Put a bare-bones Linux kernel on it booting to a shell and then have a simple binary (like mpg123) connect to an audio stream at boot and try to play it. mpg123 will happily keep connecting to a non-responding stream if given a suitable time-out.
On a server (could be another RPi, could be any computer) run Squeezebox server (formally called Slimserver) which is a free download from Logitech. Effectively this will serve a custom mp3 stream to a unique IP address from http://musicserver:9000/stream.mp3
. The Squeezebox server software automatically registers new players by IP address as they connect and you can assign them names. Obviously it includes full music library management too!!
You will need to setup the Client RPi with a very small buffer to achieve responsiveness.
Now you can control the content of the stream sent to the various RPi's from either the server PC of from a tablet or smartphone via Squeezebox servers web interface which is excellent. Squeezebox will push out MP3 audio to the custom stream and change it as you pause and shuffle tracks.
Volume control would be the only issue, but I think you could easily hack up a simple perl script to adjust ALSA volume on players from a browser. Squeezebox server might even be able to re-encode MP3 streams on the fly to change the volume if needed but I have a feeling that this would be a little too laggy.
As for getting the audio out - that's up to how much you want to spend. You could make a enclosure containing the RPi and speakers, you could embed the RPi and some speakers above a suspended ceiling or just make a simple black box to connect to a stereo via 3.5mm line out.
I'll probably make 1 x RPi-based 'boombox' and 1 x RPi-based headless unit to connect to a decent audio system. You could also take a pure digital signal from the HDMI into a suitable amplifier.