I just installed a Netgear GS716T switch today, in the middle of my network. The default settings for the switch have it blocking STP BPDU packets. This caused a network storm, as Sonos units behind the switch were not getting the BPDU's from the switch.
Turning on BPDU flooding fixes the issue.
I then turned on the switches STP. Same issue. It turns out that you have to enable STP for the switch, and enable it on each port, or at least each port that can cause a loop (the sonos connections, and connections to other switches).
Its working fine now that things are properly configured. Just posted this as a heads up.