Is the Beam connected to your TV via HDMI ARC or optical? Under Digital Audio Out, do you have any other options other than PCM and Auto? Also, be sure to turn off any audio setting where the TV is changing the audio in any way.

Currently using HDMI-ARC, as I’ve done with my other TVs. The options I have are “passthrough”, “PCM”, “auto” and “off”. Passthrough and PCM sound the same. All audio settings/enhancers are set to off. 
I have one setting that’s called Dolby Atmos, which says “only on when Dolby Atmos is activated”, so it’s been on for now, but I guess that’s Sonos Beam don’t have Dolby Atmos capability, so I’ll try turning it off.

How does it sound with it set to Passthrough? When watching a 5.1 source, does the Beam play Dolby Digital 5.1 audio with this setting?

When the setting is set to Auto, the TV is actually converting the incoming audio to the highest audio format the Beam supports which is Dolby Digital. This is what is causing the delay.

It still says Dolby Digital 5.1 in the Sonos app when selecting Passthrough, and to my ear it sounds the same as “auto”, so that’s nice. 
With Passthrough, is that the setting that the TV chooses when I have selected auto, to get the best possible audio through to the Beam? So auto is like an extra step where the audio can fall out of sync, instead of just selecting one option and staying on that? Maybe hard to understand what I wrote above, but maybe it seems like I’m getting the hang of it?

Keep the setting on Passthrough so the source audio will remain untouched by the TV. If you are watching a Dolby Digital 5.1 movie, the TV will “pass through” the Dolby Digital 5.1 audio to the Beam. Likewise if you are watching a stereo 2.0 source, the TV will pass through stereo 2.0 audio to the Beam with no audio conversion.

Thanks a lot for your help!