Even though it is a computer, it only requires a screen during installation. Then simply place it where you want an audio zone, connect the audio out to a receiver or directly to compatible speakers.
So lets go through some basic info about the Raspberry Pi and then the Raspbmc image.
Raspberry Pi, the basics
The Raspberry Pi is a credit-card sized computer running an ARM11 700mhz processor and a powerfull GPU capable of BluRay quality playback and digital audio through HDMI (or analog audio through 3.5mm). It has a ethernet port and USB 2.0. It can use various Linux distros for OS including the Raspbmc distro with AirPlay.
Raspbmc is a minimal Linux distribution based on Debian that basically turns Raspberry Pi into a XBMC frontend. No knowledge of Linux is needed and the XBMC GUI is lauched at system startup. It is installed with a few clicks from a Mac or a PC.
Raspbmc auto updates so you will constantly get new features, performance and driver updates (can be turned off). Access audio or video sources over NFS, SMB, FTP and HTTP. And most importantly, AirPlay and AirTunes support that allows you to send music and video from an iPhone/iPad to the system. XBMC itself has many additional plug-ins, for instance the virtual Squeezebox player XSqueeze. XBMC can be remoted from various apps, like XBMC Constellation and XBMC Commander.
Raspbmc also lets you overclock the processor up to 1.5Ghz.
You can get seven Raspberry Pi for the price of one Sonos Connect. You would then also have the ability to play 1080p movies and the XBMC extendability. The price/performance ratio is great. Raspberry Pi is definitely worth considering when planing a multi-room solution.