The Autoplay feature on the Line-In turns off the speaker when no audio is sensed on the Line-In.  The Era 300 is not made for output from a TV due to this, and is not recommended for this purpose.  The other reason it is not recommended for TV output is the 75 ms delay on the Line-In for buffering, which causes lip sync issues.

Sonos devices made specifically for TV have circuitry which forms a direct connection to sub and surrounds, allowing for a smaller buffer that overcomes the lip sync issue.

Thanks for the reply. The audio delay hasn’t bugged me, so that hasn’t been an issue. I tried setting things up this way since we didn’t want to look at a soundbar, and the Era 300s otherwise sound great when they are on. If there’s no other workaround I might just have to get rid of the speakers.

Based on what I read in ​@jgatie ‘s reply, you should turn off autoplay, and just use the commands in the controller to make any changes. He states the feature is associated with autoplay, and not the line in feature itself.

The autoplay in this case is pretty much just grouped rooms volume adjustments, not the home theater autoplay. Without a soundbar, the home theater menu is not available.

Autoplay is also an option for Line-in. 

No, the Autoplay feature on the Line-In is exactly like the Home Theater one on an HDMI source.  It automatically plays the Line-In source when it senses an audio source playing on the Line-In.  Which is why it cuts out when the source is quiet.

There’s no work-around for what you want. The Era 300’s are not designed to be used as TV speakers. Take ​@jgatie advice.

Apologies for my comment on the autoplay. It had been a while since I had messed with that setting, so I forgot that I was able to deselect my speakers there. I will turn that off and see if it helps. Thank you all for your help