No ability to change the balance in EQ?

I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution. There’s much more that Sonos can do with “hard data” than they can with “I hear” comments. 

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

This really could have been answered in a single reply - 'Unfortunately there is at present no balance control for a stereo pair'.

Whilst that may not be the answer the original poster wants to hear it's certainly less maddening than pointless requests for diagnostics that deliberately avoid the issue.

Just awful.

There is balance control for a stereo pair of Sonos speakers. There is no balance control for a pair of speakers set up as surrounds. Big difference.

Yes I take your point and you're right of course, I was referring to the rear surround pair in context of the original question but my exasperation reading the support response got the better of me!

Something to try: Mess with Trueplay by re-defining your listening position to be closer to the Right surround.

 I was trying to adjust the balance in my Sonos Beam surround audio system. I could find no way to do it and then went online and saw this thread. It is several years old, but just wanted to let people know this information:

 I also have several sets of Roam speakers in my home. The EQ setting for those speakers clearly has a balance slider that works between my paired sets of Roam speakers. I am not sure why the Sonos person dealing with this issue could never point out that stereo pair‘s using Roam speakers do in fact, have a balance slider. Apparently Sonos has chosen to use only Trueplay to control balance in a surround sound system. It sure seems like they could have included a balance slider to override the Trueplay in both the sound bar and surround speakers. Such a basic requirement! 
 

Left and right balance is a basic element of stereo sound. The Sonos person just didn’t seem to grasp this concept. For decades I have listened to Ian and Sylvia, a Canadian folk duo. They are each separately mic’d in the recordings. I absolutely love Sylvia’s use of harmony and often times shift the balance to focus more on her voice. Thus balance is sometimes an essential adjustment for my musical enjoyment. I also occasionally find myself sitting in a location that is not dead center between my speakers. Rather than move, it’s nice to be able to make a small adjustment to suit my needs at the moment.

For twenty years I worked in audio and video production. Another video producer I worked with developed a partial hearing loss in one ear. Without a balance slider in his mixing board monitoring system he would not be able to do the essential audio work for his projects.