SSDP discovery (M-SEARCH) gets no response from any player – not even player-to-player

Did you get a chance to check out the link that craigski posted?

https://en.community.sonos.com/tutorials-and-how-to-s-229149/sonos-unifi-best-practices-recommended-settings-6933597

Yes, went through it point by point:

Multicast to Unicast: off

Multicast and Broadcast Control: off

Client Device Isolation: off

Proxy ARP: off

Hotspot: off

IGMP Snooping: enabled (UniFi network level)

mDNS: enabled, on both the default VLAN and the IoT VLAN

All speakers are fully wireless, so the STP section doesn't apply.

Every setting in that guide already matches the recommendation. Doesn't touch the core issue either, since my packet captures show the problem persists even Sonos-to-Sonos on the identical AP, which none of these settings would explain.

If I’m not mistaken, your packet captures are of transmitted packets. You don’t know if the transmitted packets were ever received at a destination in order to generate a reply.

I’m not a network expert, but until/unless you find someone that is, you should contact Sonos where they can look at the diagnostics and possibly see where the failure is. Or help you figure it out. You’ll probably need to get past level 1 support though. Sorry I can’t be of more assistance. Maybe others with more experience can.

You're right that a capture on the wire only confirms transmission up to that point, not that the packet actually got received and processed by the destination over the air. The port-mirror test gets me as close as I can without sniffing the Wifi link itself: it confirms the query reaches the AP uplink correctly, but what happens after that hop is a blind spot. The one exception is the wired-only test with no AP involved at all, where transmission and the same-segment delivery are as direct as it gets, and even that got zero replies. Still, I take your point that I can't rule out an over-the-air delivery failure with the tools I've used so far.

I'll go the phone support route and push past level 1 as you suggest. Appreciate you taking the time to look at this.

Quick status update in the meantime: right now Bathroom, Kitchen (both on AP Flur) and Office (AP Büro) are reachable, while Living Room/Arc (AP Flur) and Bedroom (AP Büro) are not. So both APs currently have one working and one broken speaker at the same time. And none of this changed because of anything I did, no reboot of a speaker or an AP, they just came back and disappeared on their own between checks.

Edit: switching my phone to the IoT VLAN, the other two missing speakers show up immediately (routed through the firewall and udpbroadcastrelay). Switching back to the VLAN/SSID the speakers are actually on, both disappear from the app again.

Sure sounds like client device isolation on the AP.  I know you said it’s off,  but...

What device were you using for packet capture/tcpdump, and how was it connected to your network. As mentioned above, to get an accurate tcpdump you need to do this on the AP your devices are connected to on the bridge (br) interface. Plenty of tutorials online showing how to do this.

Lets rewind a bit. I’m assuming Sonos was working on your network at some point, then a change was made, and now you are missing Sonos devices on your Sonos App? What has changed? New U7 APs added to your network?

I strongly suspect there is a mis configuration in the UniFi Network App for your U7 APs, I would go back to the link I posted above, and double check the recommended settings.

You have 4 APs, so unless you have a very large house with no neighbours, you will find the channels overlapping (noise), especially if you have them set to auto. I would spend some time reviewing the channels, channel width, and radio transmit power for each frequency on each AP. UniFi Network App can help you with this in the AirView Environment section.

You mention that HA is working OK, is that using native HA or MA? I believe both use uPnP (SSDP) for discovery. Have you tried play.sonos.com?

Is the controller (phone) connecting to 6GHz band? Have you tried disabling 6GHz radio on the U7 APs for a test?

Thanks, appreciate you digging into this with me.

Capture setup: OPNsense itself (the firewall/router) and a laptop on the same VLAN as the speakers, not the AP's bridge interface directly. Fair criticism, I haven't captured on the AP itself yet.

History: Sonos did work before, though not flawlessly. I had a U6 Lite for a while that gave me trouble (I'd read about chipset issues with that model and swapped it out). Before that, I ran AC Pros for a long time with no problems I was aware of. So there's a rough correlation with the U7 Pro switch, but there have also been several UniFi and Sonos software updates in the same window, so I can't pin it on the hardware alone. I also can't rule out that the mix of old Play:1s alongside newer Era 100 and Arc units interacts badly with current-gen APs specifically.

Channels: I have 4 APs total, but only 2 of them are anywhere near the Sonos speakers, the other two are in the attic and garage. Haven't checked AirView yet, will do.

HA: native integration, not MA. HA sits in a separate IoT VLAN. Previously the speakers were on a different VLAN and traffic was routed through OPNsense; I just moved the speakers into the same VLAN as HA a little while ago.

Current state after that move: discovery works well right now, both with my phone on its normal VLAN (routed) and with the phone switched directly into the IoT VLAN. That change is only a few hours old though, so I want to see how it holds up before reading too much into it, given how this has gone with previous changes.

play.sonos.com: hadn't heard of that, thanks, will try it if the problem resurfaces.

6GHz: good call, hadn't checked that. If things go south again on the IoT VLAN, I'll switch back to the original VLAN/SSID and pay attention to whether the phone is on 6GHz, or just disable that radio for a test. Could also connect the phone to one of the AC Pros instead for a cleaner comparison.

I'm 100% sure it's off. I'd be thrilled if it turned out to be just a small setting somewhere.

Update: no issues on the IoT network so far. Not convinced it's band-mixing specifically, but the IoT SSID is 2.4GHz only and configured differently overall, so it's possible the speakers just don't get along with some setting that's active on the main SSID. If it stays stable, I'll start comparing the two SSID configs setting by setting.

Does this latest update in the UniFi: Best Practices tutorial help?