If you’re running a single Era 300, that means the speaker is grouped with the Beam, and not set up as part of a surround speaker pair. The line in on the Beam tends to be ‘fatter’ than streaming music, which makes the connection between the Beam and the Era 300 more sensitive to wifi interference . I’d read that FAQ and apply as many of the solutions presented as possible.

If it continues after that, I’d 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 network and Sonos system.

Goto the Beam ‘room’ settings and increase the buffer level in ‘Group Audio Delay’ - if that stops the dropouts, then it’s an issue with your local network not-coping with the demands of the TV audio over the LAN. To ‘perhaps’ begin to improve things you might be best to go onto look at the suggestions in this Sonos Support document…

https://support.sonos.com/en-us/article/reduce-wireless-interference

Ty