I would take it to support, by submitting a system diagnostic within 10 minutes of experiencing this problem, and calling Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Play something and after 10 minutes submit a diagnostic then call support. Calling seems the most effective option.

There are multiple potential aspects to this issue. Obviously, one of the speakers could be defective. Physically swapping the speakers will give some insight into this possibility. The physical location might cause one speaker to seem quieter. (The human is easily fooled!)

And your choice of music might be in the mix of potential problems. At some point you should “Remove Stereo Pair” then reverse the L/R assignment as you pair them again.

You could also unpair and keep the speakers as separate Rooms, Group them, then play music. If the speakers are physically close to each other, the output should be very similar. If the output is very close, but not identical, you can physically swap and test again.

There are also some free or cheap phone Apps that can measure audio levels. Don’t expect these Apps to be professional level that you could use to publish research papers, but they will be fine for our immediate purposes. You can also find various test tones and play them through the speakers. Unpair and play the tones on each speaker noting the results. For these tests, the speakers should be in exactly the same location, orientation, relative position, and distance from the measuring phone. Clamping the phone to a chair would be a good idea. The measurements will not likely be exactly identical, but they should be very similar. Make sure that both speaker’s Volume, Tone, and Loudness settings are identical. Play only one speaker at a time.

Have you updated the hardware and the app? If not do so. Updating the hardware requires you to go to settings and general and click on software updates. Do this even your speakers did an update upon installation. After that…update the app by deleting it and reinstalling. Select join an existing system accept all prompts and sign in with your credentials. If those steps resolve your issue then all good. If not follow the advice to submit a diagnostic.