That sounds like a failure in the CEC system in recognizing the Beam’s ARC connection consistently. My bet would be on the Philips side, but my personal opinion of their ARC/eARC implementation is low. It’s probably safest to reboot both devices. Which means unplugging them from the wall for two minutes. First, plug back in the TV, wait a minute, then plug in the Beam. 

Check the TV and the Sonos for software updates, and apply them. Then check to ensure CEC is turned on in the TV’s settings. 

For what it is worth, if that doesn’t fix the issue, I’d call Sonos Support directly to discuss it.

​@BrionF 

I agree with ​@Airgetlam and just can add my advice to check your tvs firmware for an update. Sometimes updates are not found automatically when checking online from the device itself. I recommend to look for an update directly on Philips support pages on the web. 
Another thing I would give a try is using a different hdmi cable. 

Thank you for your answers.

I’ll try this weekend and give you a feedback.

Hello,

I tried the reboot thing. So far it seems to be working. 

Thanks again.