My Wife and I have the Sonos Beam (with surrounds) in the Dining Room and the Sonos Arc (with Surrounds) in the Family Room, and I think the dialogue (audio in general) from both setups, are pretty similar overall.

Each setup is connected to an LG TV (different models) and so just wondering if it’s not your TV, have you perhaps considered speaking with Sonos customer support to see if there might be an issue with your Soundbar, assuming it’s not a TV compatibility issue.

I think it’s even worth video-recording a sound comparison, if it’s that obvious a difference, aswell as submitting diagnostic reports, just to try to show the issue and then let Support Staff see if there is anything further they can do.

Personally speaking I think the Arc sounds great, but I guess your setup could have a compatibility issue with its connected TV, or perhaps even a fault.

It is not the TV. I have a Bose 900 and their dialogue mode works perfectly, a very noticeable difference. The speech enhancement on the Arc simply docent work. 

I hear a subtle difference when toggling speech enhancement on and off, but it’s ‘less’ of a change than it once was, most probably because the Arc dialogue has noticeably improved in the recent software updates.

It’s more noticeable in my case when Trueplay is disabled on the Arc room, but for some, it might ‘perhaps’ (unlikely) be the other way around, but the feature is working here for me.

Night Mode is also working and much more noticeable. I do use night mode each evening so as not to trouble the neighbours, but rarely find the need to use speech enhancement, particularly since the software changes. I guess things may vary by setup/environment, but I can’t comment without actually hearing things side-by-side and that’s not likely going to happen.