Same issue. Arc Ultra with Era 300

 Rear channel sound coming through the Arc Ultra.

Does anybody know if this is an issue with the original Arc? I would like to just return the Ultra and get the original. But if it’s got the same issues I will be returning  the whole system.

I guess it was never an issue with the old Arc, because it is a 5.1 channel soundbar. Before I had the Beam Gen 2 with 5.1 channels also. It was never an issue and the surround sounded always correct from behind. In many cases the Beam Gen 2 had a much better surround experience (because of this issue).

I had the the AMP (the newest one), that i upgraded from……… when i plug back the amp and use that as the main + pair to surrounds it works as expected.

No surround in the coming out in front for DD 5.1 content.

To add to both I think the idea was to compete/compensate for a true 7.1 surround sound being left front,center,right front,left rear and right rear and compensating for a left surround and right surround with signals coming from both front and rear left side and right side channels.

I would rather have the 5.1 then 7.1 FX processing 

At the moment, after endless testing, I am 100% sure that the Arc Ultra together with Era 100 Rears continues to calculate internally with 9.1 and does not switch to 5.1. If you look at the speaker arrangement of a 9.1 surround system, the 4th and 5th channels are positioned diagonally at the front (red circles).

Only the 6th, 7th, 8th and 9th channels are then present at the rear. And this is exactly what my sound system sounds like. The 4th and 5th channels come from the side at the front and there is silence at the rear, but if you then switch to a better soundtrack with more channels, the sound comes correctly from the rear. With a 5.1 soundtrack, however, the 4th and 5th channels must always come from the back. This does not seem to have been programmed correctly with the Arc Ultra and Era 100. I hope that Sonos fixes this serious error with high priority. An error like this simply cannot be allowed with a 5.1 soundtrack!

Has anyone tested the exact same song in Apple Music atmos through the Sonos app vs through the Apple TV app? Perhaps that could clarify the issue

I‘ve found no difference between the Sonos App and Apple TV 4K with Apple Music and Dolby Atmos. Both seemed to work fine. In my opinion this issue exists with 5.1 Soundtracks only. Because during 5.1 Soundtracks the 4. and 5. channel speakers have an other position in a room than with Dolby Atmos, 7.1 or 9.1 Soundtracks.

I see, sorry I misunderstood. Makes sense 

At the moment, after endless testing, I am 100% sure that the Arc Ultra together with Era 100 Rears continues to calculate internally with 9.1 and does not switch to 5.1. If you look at the speaker arrangement of a 9.1 surround system, the 4th and 5th channels are positioned diagonally at the front (red circles).

Only the 6th, 7th, 8th and 9th channels are then present at the rear. And this is exactly what my sound system sounds like. The 4th and 5th channels come from the side at the front and there is silence at the rear, but if you then switch to a better soundtrack with more channels, the sound comes correctly from the rear. With a 5.1 soundtrack, however, the 4th and 5th channels must always come from the back. This does not seem to have been programmed correctly with the Arc Ultra and Era 100. I hope that Sonos fixes this serious error with high priority. An error like this simply cannot be allowed with a 5.1 soundtrack!

Thank you for all your work on this! I am new to Sonos! Is this something you think they will address or are we out of luck? 

Yesterday I had contact with Sonos Chat support in Germany. Seems they are aware of this issue. This is the transcript (translated from german to english):

Sonos Support: Hello. My name is ******. How can I help you? 

Me: Good afternoon ******, I'm having problems with my new Sonos Arc Ultra soundbar with Era 100 rear speakers. Basically the sound is great, except with 5.1 soundtracks, then the sound from the 4th and 5th channel seems to come from the front side instead of the rear. It seems as if the Arc Ultra, together with the Era 100 Rears, does not route the 4th and 5th channels to the rear, but continues to work as with a 9.1 soundtrack. This is because the 6th, 7th, 8th and 9th channels would then correctly come from the rear, and the 4th and 5th channels would then be positioned correctly at the front and side. With 5.1 surround, however, the 4th and 5th channels should always be at the rear. Better soundtracks with more channels, on the other hand, are always played correctly. In various forums I can find lots of other people with exactly the same error who use the Era 100 as a rear speaker with the Arc Ultra. Is Sonos already aware of this error?

Sonos Support: Good day to you, ******! I hope you are doing well. 
Thank you for describing the issue you went through. Please allow me a moment to check my end.

Sonos Support: Thank you very much for your time. It looks like you are affected by a back “over performance” coming from the Arc Ultra. Thank you for reporting this to us. We are currently investigating this experience, which will be flagged in case # ******** so our team can be aware and investigate this matter. 

Me: Thanks, for now my old Beam Gen 2 sounded better in many cases. The sound always came correctly from behind at 5.1. 

Sonos Support: Thank you for letting us know about this. I have logged the case according to the issue you are facing. This is currently under investigation. 

But it is important that everyone with the same problem also reports to Sonos so that they know that there are more people with the same problem. Then there is a better chance that this bug will be fixed soon!

Moderator Edit - Removed case number.