That’s odd. The PLAYBAR, which I’ve owned two of, does not have a microphone built in to it in order to change any volume based on a fan.
I’m wondering if you’re experiencing more of a wifi interference issue, although honestly, with a digital signal, it should be either on or off, not to mention the fact that when it’s playing the TV content, it’s completely coming across the optical cable, and there shouldn’t be any interference at all.
I would recommend that you submit a system diagnostic within 10 minutes of experiencing this concern, 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.