So the issue is it's sending out BPDU packets over wireless, through the AP, through the switch, over the wired connection, to the other sonos (and vice versa), and it needs to see that happening. However the brocade is dropping the BPDU packets so they never make it there. this is my best guess anyway, from reading their stp thread but never touching sonos in my life
Try this, at the global config level:
I've never used this command personally, but that *should* allow unfiltered BPDU forwarding. you'll probably have to turn off all the sonos and bring them online one at a time after making that change.
Switches are *supposed* to pass BPDU traffic unfiltered when STP is configured OFF (as to not interrupt other STP topologies), so that's surprising that it was still happening with it off globally and at the port level