Stereo pair of Play:5s: one louder than the other?

Have you tried listening to each individual driver to see if all are working? Check both horizontal and vertical orientations. 

A paper towel center roll can help with that.

Unbond and recreate the stereo pair?

I would reboot the speaker at some point. Play some audio test tracks.

Try unpairing them and set them up as a group. Do they play at the same volume in that configuration? Re-pair them but switch so the previously left speaker is now the right. Does the same one play louder, or has it switched? 

Could be the English ‘power’.

I’m teasing you ;) You shouldn’t be visiting the old country ;)

My true suspicion is along many of the suggested options of previous posters, i.e. a problem with the electronics on one of the two devices. Which would likely be more clear by the tests previously suggested, or a call to Sonos Support to discuss it.
 

Which, for power users like you is likely an anathema, and annoying as we don’t have access to the data in a diagnostic…although I’m not 100% certain that all electronics failures show up there. My supposition is, given the age of the devices is a potential issue with an amplifier, but given it is still working a bit, it may not show up. Probably not associated with the color ;)

I tried all these suggestions, but it seems like a mechanical failure in the white one. As I only use these for my PC (ie Teams calls) I adjusted the balance to move more to the white side. Would be a terrible solution for music, but that’s ok, I use my S2 devices for that.

I recently bought a Play:5 gen 1 to make up a stereo pair. I bought it on eBay where it was described as spares/repair with low volume. It was noticeably quieter than the other half. I unpaired them and instead grouped them to verify the volume difference. The other symptom was that there was an audible pop when powering it up.
I read that these symptoms indicate a problem with the driver capacitors. I opened it up and saw that all the driver capacitors were noticably domed. I replaced them (tricky) and it now sounds identical to the other half of the pair.

First thing I’d do is swap the stereo pairing so the white speaker becomes the right channel and see if the lower output follows the speaker or stays on the left channel. If it follows the speaker, you’re likely dealing with a failing amp section or aging driver/cap issue in that Play:5 not uncommon on Gen 1 units at this age.

It follows the speaker, confirming the hardware diagnosis.

Any chance it is a bad individual speaker, they are cheap on ebay.