It seems to me you have already tried most of the troubleshooting steps, apart from an actual factory reset of each device, but before suggesting to try that, it might be best to first submit a system diagnostic report immediately after you next encounter the issue and make a note of its reference. Then contact/chat to the Sonos Support Staff via this LINK, to perhaps see if they can glean anything from your report.
 

If you do decide to go it alone and factory reset your devices, then ensure you only do one speaker at a time and get that speaker added back to your existing system and working, before resetting the other one.

Is the drift something you perceive from the volumes, or does the balance slider in tbe app actually move off-centre?

Thanks, Ken. I took your advice and just finished a chat session. My case has been bumped up to the next level. I’ll be following up tomorrow with a call to phone support. We’ll see what happens!

The balance slider doesn’t move. On tracks with a clear phantom center, where the kick drum, bass, and lead vocal start at dead center, I can easily hear them drift off to one side. It’s a shift in volume.

Are you able to experimentally connect the two speakers together by Ethernet, and see whether the problem occurs?

I might be able to do that. What do you think that would that tell me?

It would depend what happens. I have never heard of a similar issue before, but I suspect it's just a weird effect of wireless communication glitches. It the problem disappears when the speakers are wired then that would support that hypothesis. 

Sonos 2nd level support told me I have too many speakers without having one wired (9 total). I’m adding a Boost to see if that solves it. I’ll report back here either way. 

Just another update. I took all of my Sonos products offline, rebooted my router, factory reset both One SL’s, set them up as new, re-paired them, and re-did Trueplay tuning. All of this before bringing the other speakers back online. Same problem as before, no change.

My Boost comes tomorrow. If that doesn’t solve the problem (doubtful now), I’m going to assume one or the other of the speakers is defective.

Just curious, but did you try @John B’s suggestion of tethering/wiring the two speakers together to see if that made any difference? 

I haven’t. My router is upstairs so I’ll have to move them, then set them up in way that I can clearly hear the stereo image. A bit of a hassle, but worth trying. I’ll try that as a last ditch effort if the Boost doesn’t solve it. 

I keep having this nagging thought. Does anyone have any theories why this issue crops up using the Sonos app, Tidal Connect, and Spotify Connect, but not when using Airplay?

Different end point servers? Which opens up all sorts of transport issues, such as DNS. As well as bandwidth difference possibilities. 

All of the other choices are from an internet server to the speaker, without your phone being involved. AirPlay 2 is from your phone to the speaker, so it uses whatever server your phone is being told to contact, which often is not the same as what Sonos (the speakers) have been told to reach out to. 

I’d certainly be thinking about submitting a diagnostic to Sonos directly, and asking them what they see in the data. Which I assume you’re doing anyway, since they’re bumping to level 2 support. 

But John B’s test, even temporarily, would certainly help identify things,  if it was a bandwidth possibility. Probably why he suggested it. He’s a pretty smart cookie. And the nice thing about Sonos...not hard to move around the home temporarily. 

Not sure about the Airplay thing, but you could connect one of your other speakers ( not from the troublesome pair) by Ethernet to your router.  This would  do the same as the Boost will do 

Wiring the two units together without putting the system into SonosNet mode will do nothing. The Ethernet ports are disabled in WiFi mode. 

Loss of sync like this tends to be associated with a rare hardware fault. In that case there will be clear evidence in the diagnostic's logs. 

The tech I spoke with yesterday looked at my diagnostic and didn’t mention anything about a hardware fault. That does seem likely to be the issue though, considering I have three other stereo pairs on the same network that aren’t exhibiting this behavior. 

Any of those a pair of ones? You could then swap one of each and see if that solves it, or to identify the defective one

That’s a great idea. Unfortunately, I have a pair of Play: 1’s but not another pair of One SL’s. I could buy a single One SL and do that though, which might be worth it. If I identify one of them as being the problem, I could see about getting it replaced or at worst use it as a filler someplace else in the house. I’ll have to consider that. 

Just following up here. I ended up purchasing a third One SL and used it to test against the existing stereo pair. I swapped it out with both of my older One SL’s and in any combination the stereo centering issue is still there. So it’s not the speakers.

I also purchased a Boost and hardwired it to my router, then checked to be sure that everything moved over to SonosNet (it did). That also didn’t solve the centering issue, though so far it did seem to eliminate the occasional dropout I was seeing. 

At this point there doesn’t really seem to be a solution that doesn’t involve replacing my modem/router, which I’m unwilling to do right now. I guess I’ll just use Airplay which continues to work fine.

Thanks for all of your suggestions. Great community here!

I decided to make one more effort to get to the bottom of this today. I called in to support with my case number and got nowhere. The support person actually wanted me to take a video of my speakers with my phone and send it in (SMH). Of course, the phone video would be mono, which would make it worthless for troubleshooting a stereo issue. Reaching a point of frustration here. 

Are there any Sonos support staff active on this forum? If so, can you take a look at this diagnostic and let me know if you see anything strange? This was captured when the issue described above was happening.

Diagnostic #939061778

TIA. 

Have you tried physically relocating the speakers to a room where another pair is working normally?

I can't imagine what environmental factor might cause your problem, but it would be a quick and easy experiment. 

Also, have you tried connecting the speaker pair by Ethernet while in SonosNet mode?

Hi John,

I haven’t done either. Relocating to another room would be simple, I will do that. I only have two Ethernet ports on my router, one of which is connected to my Boost, so I guess I’d need to get a switch to make that work. 

Thanks..

I just want you to wire one speaker to the other. I am not optimistic it will make a difference though.

OK, got it. Will try it…

OK, tried the speakers in another room (closer to the router), with and without the Ethernet cable between the two, no change. The stereo image is still drifting to the side when streaming any source other than Airplay. I’m stumped.