That does sound fairly odd, I suspect it is a bandwidth issue with the wifi, inasmuch as only some amount of data is reaching the speakers. I’d be curious if you can reproduce the issue while having one (or both) sets of speakers wired with an ethernet cable, too….that would be a good way of checking if it is indeed a level of wifi interference.
But without Sonos looking directly at some hard data, it’s extremely challenging to guess. I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.
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 phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.
Thanks Bruce, will contact Sonos!
Keep us posted, it is always interesting to see if the “guess” is correct And good luck!