As mentioned in the first page on this topic there are a number of alternate ways of implementing an open source music player that performs similar to Sonos.
SqueezeBox is a Logitech solution that has a number of the components open source. It is a music streamer so music on a Squeeze Server can be streamed to a number of SqueezeBoxes.
Squeezebox is a network music player from Logitech (now discontinued) but there are a number of open source implementations that can be used.
Logitech Media Server (LMS) is the server software that powers audio players from Logitech (formerly known as SlimDevices), including Squeezebox, Squeezebox Boom, Squeezebox Receiver, Transporter, Squeezebox2, Squeezebox and SLIMP3. Logitech Media Server is the new name for Squeezebox Server, starting with version 7.7. Before it was known as Squeezebox Server (version 7.4 to 7.6) and SlimServer (before 7.4).
Logitech Media Server is Open Source Software and written in Perl. Logitech Media Server runs on pretty much any platform that Perl runs on, including Linux, Mac OSX, Solaris and Windows.
It can be downloaded from here http://www.mysqueezebox.com/download
Logitech maintains a good wiki at wiki.slimdevices.com/index.php/Logitech_Media_Server and wiki.slimdevices.com/index.php/Main_Page
Installing LMS is well documented on the wiki and various other posts so I will not go into detail here. I installed this on my Linux server that I run MythTv on, this server is always on so ideal for music anytime.
After installation you can access the LMS on the server using a web browser at port 9000. You will need to open port 9000 on the server firewall to access. Setup is simple, just enter the location of your music and LMS will scan the directories. The mp3 tags are read and the database is created. I have a file in each album directory called folder.jpg which is the cover art for each album. LMS reads these cover art files and displays in the user interface.
There is also a great iPhone app to select and play the music from Logitech and it is free – called Logitech Squeezebox Controller. This does the job, a better one exists, iPeng, but I have not tried it.