Here are a few considerations to make Sonos speakers work on an UniFi meshed wifi network.
Do not block multicast transmissions (uncheck Block LAN to WLAN Multicast and Broadcast Data). Disable the UniFi Auto-Optimize Network feature. However, enable ‘Point to Point‘ and ‘P2P Cross Connect’ in the wireless network settings.
Wire connected Sonos devices will create a SonosNet to transmit over a ‘Sonos only WiFi’ with other Sonos devices (Sonos Move & Roam excluded). You may observe in the UniFi Network clients list that the Sonos wireless connected speakers are not ‘visible’, except for the Sonos Roam or Move, while all Sonos wire connected device are shown. This may be related to the SonosNet (mine on channel 6) connectivity. You may notice in the Sonos Controller that all Sonos devices (wired and wireless) have an IP Address with WM:0 except for the Sonos Roam (in my case using 11ac (5GHz) – channel 40) that has WM:1.