If you have a voice assistant installed/connected to your Sonos system, then it’s ‘wake word’ might be being picked up by something mentioned within the TV audio output.

"A letter has arrived” can be mistaken for “Alexa, has arrived” …and that may trigger the voice assistant… just as an example.

If that’s the case, the wake word will duck the playing TV audio for the assistant to listen-out for an instruction.

Anyhow, as an initial test, if you are using a voice assistant on devices in the Home, then perhaps see what happens if you mute their mic temporarily and just see if the issue goes away. 

Sounds like ‘ducking’ for a Voice control command. Have you double checked that your Beam has voice control turned off, and that it isn’t part of any ‘groups’ in voice control software (like in Alexa, not Sonos).

If it isn’t, you should reproduce the issue, then submit a system diagnostic, 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 network and Sonos system.