Best answer by Chris
If you don't then the next best thing is unplug turn off as many network devices in your house as you can to get them all off your router. Reboot router and then turn back on your network devices. Hopefully you turned of the offending device that is holding on to an IP address it shouldn't and messing up your sonos.
If you don't find and turn off the offending device then it will keep messing up the IP address even after router reboot.
That's why the permanent and best solution is just give your sonos units new fixed IP addresses of their own that other devices then won't try to take.