Regarding realtime stuff there's a project out there to implement the Ethernet AVB (audio/video bridging) standard on BeagleBone using PTP (precision time protocol) for synchronization.
Some of the older network synchronized transports like CobraNet and Dante might also be interesting for anyone wanting to learn more about this stuff.
If you ever get around to adding this I will start building my el cheapo raspberry pi based sonos clone. ;-)