I do have the same issue. Using eArc and my Mac, I was able to test every channel through audio configuration and the center one is really too loud when we compare to right and left ones. An update, or a setting to adjust it would be really cool. 
Right now it is sound worse than my previous Gen1 beam, that’s a shame for a 999 sound bar :’( 

Hi @not_convinced & @mindthings 

Welcome to the Sonos Community!

Sorry to hear that you are not both entirely happy with your new purchases. 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 - if there is an issue with the sound mix, it will need to be documented before it can be addressed.

I hope this helps.

Totally agree, the centre channel is a few decibels louder than the left and right which obviously helps with dialogue but ruins any stereo effects. I really hope Sonos fixes this or at least give us a centre channel slider to allow user adjustment like the surround level one. This is not a user error as Sonos tech will try to convince you, this is definitely a simple software issue which can be solved very simply with some in house adjustments. In order to convey a convincing surround effect it is essential that all levels are matched for each channel.

What are your results when you select different levels of speech enhancement?

I leave speech enhancement off as it makes it even worse completely drowning out stereo effects and causes some awful sibilance.

I'd get the problem to appear and submit a diagnostic, then call Sonos to get them to look into it.

I completely agree that the center channel is too loud on the Arc Ultra. Using the Dolby Atmos test tones video, the center channel is 4-5 dB louder than the front left and front right channels.

I listen to music mixed in 5.1 on Blu-ray and it is really noticeable. Any vocal or instrument that plays out of the center channel is far too loud and makes the audio mix sound unbalanced. It would be helpful to have a separate center channel level setting in the Sonos app.

One of the reasons among several others why I ended up returning my Ultra in favor for an Arc. In my opinion the Arc Ultra was quite a significant downgrade compared to my other Arc-based system.

What do you see when selecting various levels of speech enhancement on the Ultra?

I have noticed I can hear dialog on my Ultra, set to medium that I couldn’t understand on my Arc. Haven’t played much music on it to have an opinion there.

I have this same issue. Just bought a Arc Ultra, and Sub 4….and the center speaker is WAAY over powering...and I hear no left or right. It’s just very light ambient sounds.  I keep compariing to my Beam + 2 Play 1s...and I’m really disappointed. Do I need to buy Era 100s to get better left and right?

As I asked the previous poster… Have you tried the various levels for the center?

I did. I truly can’t tell the difference. And even question if it’s on. Is there a movie or show where it really shines? 

Odd, I notice a big improvement over my Arc using the medium setting, the low is better than the Arc but not much. Only used high on a couple movies where the dialog was pathetic and needed a lot of help.

Aren't you noticing a big difference in the settings?

I noticed the same with my new Arc Ultra. At first, I was very pleased with the clarity during dialogues, but now I have noticed with many films (Atmos or D5.1) that in scenes where the music or effects should be in the foreground, they come across very quietly. I used a Dolby test video to listen to the individual speakers, and yes, the left and right front channels are much quieter than the center or even rear speakers (tested with and without speech enhancement). Is this intentional on Sonos' part? I wonder why I bought a $999 soundbar when my built-in TV speakers convey more “action”. It would really help if you could adjust the volume of the center and front speakers in the app.

I watched Ready Player One on 4K UHD last night. At lower volumes the center channel dialog is quite a bit louder than the rest of the audio. For more balanced audio from the front soundstage, the volume needs to be increased but not too much as the dialog will begin to be uncomfortably loud. So there is a very small volume range where the audio sounds balanced in my room.

The center channel being 4-5 dB louder than the front left and front right channels is a problem. The ability to lower it 1-2 dB would be just about perfect in my setup. This is why a separate Center Channel Audio level setting would be incredibly helpful. If this setting was added, it would be extremely popular and a big win for Sonos and their users.