Hi

At this point you have two options I’ll list in order of preference:

1. Upon the next occurrence of the audio imbalance within 10 minutes run a diagnostic and post the reference ID in your next post. Call Sonos tech support to discuss the findings in the diagnostic.
2. Break the stereo pair. Factory reset the speaker that isn’t performing as it should. Set up the stereo pair again. If the issue is resolved then all good. Note: A factory reset will erase any information that Sonos might have used for a diagnostic. If the issue isn’t resolved then you are back to square one. Definitely follow the suggestion in one (1) above.

It maybe best to undertake playback and try toggling/adjusting both Trueplay and EQ settings and then if still no joy, having unpaired/paired the devices, perhaps immediately submit a Sonos system diagnostic report from within the Sonos App, note it’s reference and then contact/chat with Sonos Support Staff via this LINK and see what the Staff can perhaps suggest to resolve the matter. 

They may go onto advise a factory reset of one, or both speakers, (doing just one speaker at a time and adding it back to the system before doing the next speaker) but before doing those tasks, I would speak to Sonos Support Staff first.

Thanks. I have just submitted the diagnosis. 
 

1164492206

@Jonathanjames 

You can also try to set them up again as stereo but swapping the devices for left and right. This could help to indicate a hardware issue if „the problem“ also changes left and right. 

Remember you have to call support to get them to look at the diagnostics, otherwise they just sit in the Sonos file server.

Sometimes the forum staff can look at one but it isn’t their primary function and they may lack the full set of tools support has.

I swapped them over and re paired and the same speaker now on the right is still louder. I do have another one sl in the post which I will try and see how that works out. 

And to do one more test on hardware issue you could set up the speakers each as a single device room and compare them. If the one that sound with a lower level also does so after a reset and new setup as a single device, imo a hardware issue might possible. 
After running these tests Sonos support would have much more information to help you. 

I have also done that and sure enough the one is still louder. 

If you turn Trueplay off, with the balance slider set to the centre, is one still louder than the other? If not it would seem to be a Trueplay anomaly. 

Yes tried that and true play on or off is the same unbalanced sound. Both speakers sound fine on their own so must be a software issue?

I think I’d try deleting the app from your phone and then reloading it. Also check all speakers are on latest system update. Or hold off and see if Sonos see anything in the diagnostics report. 

So if you have done a reset to both speakers, both are on latest firmware, both set up as a single device room, both trueplay off, eq and loudness settings the same and the One SL isn’t as loud as the One, I would assume a hardware issue. 
Maybe Sonos support can see the real amp power in a diagnosis. 
Please call them and do some investigation. 

Sonos are sending me a replacement as they can’t fix it. A search online reveals others have experienced this so what are the chances this is going to work?

Imho the chances are quite high that you will be satisfied with the replacement. The One and One SL are some of the most sold Sonos products and I don’t know the number of pruduced items in detail, but in relation to the few ones being affected by such a hardware issue there’s a very high quality standard I think. 

I can’t remember what the Sonos rep said now about how the return works, do they wait for it to be received before sending a replacement?

Depends on whether you have added a credit card as payment method for your account or not. If so, the replacement will be shipped as soon as possible. Otherwise Sonos will wait until your device arrived.

You should got emails from Sonos with instructions how the replacement works and with shipping return label instructions. 

Replacement received today, and same issue. The one SL is quieter than the one. Maybe they shouldn’t be paired? Have tried trueplay on and off and no different. To get equal sound I have to move the slider to +5 towards the one SL. 
 

What do I do from here?