Stream Audio through Wifi Mesh network?
How about something like the Raspberry Pi?  Apart from running on 5v (not 12v) it looks about right for you.

This thread in the Pi Forum includes discussion on using a Pi as part of a mesh network and some details of adapters that do (and don't) work for the task.  The Pi already has a built in 3.5mm mini jack for stereo audio out and 10m between nodes should be achievable in many situations.

Then for software you've got a range of options and can use a normal pc as the source using something like VLC to create audio streams which Pi's can latch onto, have them set up as receivers and push audio to them, or just have the Pi's pull in files from a shared folder.