Try this:

Disconnect everything from the TV and unplug the Beam and TV from power for about a minute. Plug everything back in but keep everything disconnected. Run TV Setup in the Sonos app and follow the app instructions. When the app instructs you, connect the Beam to the TV’s HDMI ARC port using the Sonos-supplied HDMI cable. After the Beam has been successfully connected, connect any other devices to the TV.

I tried doing this, but the issue remains the same. Interestingly, the problem mainly occurs when playing YouTube through Apple TV (4K) connected to my Phillips TV. When starting any content there, the sound gradually mutes. However, if I adjust the volume in the Sonos app on my MacBook and keep the app open, the volume stays at the set level. As soon as I close the laptop, the problem starts again

Hi ​@yzerman001 

Welcome to the Sonos Community!

Sorry to hear of this issue you are having with the volume on your Beam Gen 2 while playing YouTube content.

I have noticed on my own system that after a while of not playing, the YouTube app on my nVidia Shield Pro will be muted and will ramp up the volume in the first few seconds of playing something again. It never used to do this - I’m fairly sure it was with a recent YouTube app update that this started.

It sounds different from what you describe, however - I never hear the volume reduce by itself.

I recommend you get in touch with our technical support team who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports. Please recreate the issue and submit a support diagnostic before calling in.

If your Philips TV has a YouTube app, you might want to try that to see if it is any different.

I hope this helps.