Keeping things simple at this stage, I would first try power-cycling the speaker and if no joy, then try power-cycling the router too and see if that solves the matter.

Thanks! 

Forgot to mention, ive tried to power-cycle the speaker. Waited in one case at least 30min. before powering it on again.

At first I was suspecting that it did not get power (as the status light is off). So I tried multiple power outlets in my home. Only later I found out (in my unifi controller) that the sonos one is connecting to my access points, such that it is not a power issue. 

Based on that the One is connected and reachable, I would expect that it is not a network issue, but I could be wrong. Will try to reboot the router as well. 

If using Unifi Wifi system, have you enabled ‘multicasting’ for device discovery? I think the steps to do that are as follows:

• Log into the UniFi controller.
• In the Settings tab, click Wireless Networks.
• Click Edit next to the network SSID.
• Expand Advanced Options.
• Uncheck Block LAN to WLAN Multicast and Broadcast Data.
• In the Settings tab, click Sites.
• Disable Auto-Optimize Network.

Please also enable IGMP Snooping (which is in the Networks settings page).

Note: the above steps maybe slightly out of date with newer firmware updates, but perhaps there’s enough there to assist you resolve the matter - if you can kindly repost any changes to the steps that will help me too.

It maybe these are now the steps (depending which firmware you are using)…

• From the Unifi Network Application interface, click on the Settings icon from the left side menu.
• Click on the Network tab. 
• Select the network Sonos is connected to. Click on the Default network name if there is more than one. 
• Scroll down to Advance Configuration, and click on Manual.
• Check IGMP Snooping and Multicast DNS. 
• Click on Apply Changes and allow up to 3 minutes for the settings to update.

Im having the exact same issue with my Sonos move. Was working fine earlier. All my other Sonos speakers are working fine. 

I checked multicast management. 

The steps I took were (in 7.3.76):

• Log into the UniFi controller.
• In the Settings tab, click Wireless
• Click on the network SSID.
• Set Advanced Options to Manual
• Uncheck Multicast Management and Multicast and Broadcast contro

In my case both were already unchecked. 

Regarding:

• In the Settings tab, click Sites.
• Disable Auto-Optimize Network

do you mean nightly channel optimization? This was on my in my case, so I disabled it. 

If so, this is now located in: Settings → Wifi → bottom of the page. 

Ive enable IGMP snooping and multicast DNS as you described. Not sure whether this is relevant. But I am not using an Ubiquiti router. 

After doing this and resetting the Sonos One by unplugging - waiting for 15min. - plugging in the power cord again, the device is no longer visible on the network.. I do notice a sound after a plugging in the One in the power outlet after 1-2 seconds (sounds like a kind of relay switching?) I cannot recall I heard this before.

You definitely need to have ‘multicast/broadcasting/IGMP snooping’ enabled, as Sonos uses that for SSDP device discovery across the LAN - so it should be enabled. I don’t use Unifi, personally speaking, but have seen the instructions I posted, mentioned here in the community, but it seems at some point the user interface was changed in the Unifi interface, so not quite sure which instructions may apply, but Sonos controllers do discover the devices using this method. So I hope that info. may assist you.

Ok clear. Thanks for the input. The thing is it worked for years but only now I noticed one out of my speakers stopped working as described in my original post. The others are working perfectly. So I would not expect it to be a specific networking setting. 

Ah okay. Is it actually showing up in the Sonos App in ‘Settings/System/About My System’ device list? Whist there also perhaps check to see if it is updated to latest version of the Sonos firmware? 

Have you tried wiring it to your router, just to see if that may make a difference and will then play? Just need to establish what you’ve tried so far. 

Ubiquiti issues frequent firmware updates, updates to the user interface, and updates to fix the updates. Overall, the units are very capable and effective, but the updates can be a pain.

There is a recent option that allows you to fix a client to a particular access point. This is helpful for SONOS if you also fix the channel for the access point. SONOS doesn’t like access points that change channels. Also, use 20MHz 2.4GHz channels.

Thanks. Since I tried te re-powering the device after making changes to the wifi settings I cannot see it anymore on my local network.

Status light on the one is not coming to live. It seems dead, but when wiring it to my switch it indicates a 10mbps connection. Does not seem to be right but could mean the unit is powered. 
 

any idea what I can try further or did it turn into a brick…?

Maybe try a power cable/socket from one of your other ‘Ones’ and see if it powers on with that - you should see a flashing status light on start-up. If no luck with that then perhaps try a factory reset and see if it springs back to life.

You can also speak with Sonos Support Staff via this LINK  and maybe see what the Staff can suggest to hopefully resolve your issue.

Thanks. I will try to contact sonos support. I tried different cords already and also measured whether there actually a voltage at the cables. That is all fine, so issue is at the sonos.

btw. I accidentily hit answered button instead of thumbs up. Seems that this cannot be undone..