Possibly a speaker failure? Are all the drivers making noise?

Is it just one source or all of them?

You could play something and then submit a diagnostic to Sonos. Call Support and see if they can see a Sonos issue.

Have you changed anything? What source are you playing?  How is CONNECT:AMP wired to the speakers? Is there a Volume control in the speaker wiring?

perhaps it’s a failure. When I move the balance all the way to left or right I get decent audio but right in the middle at zero where I assume it should be for a stereo speaker the audio is terrible. 

Nothing changed. Worked perfectly up until the day before the problem. This is a new system only recently wired in a new build. 

Until proven otherwise I'm assuming this is a wiring error. Some types of music will be more sensitive to this error than others. In other threads you noted poor sound and fuse blowing. This is another symptom of incorrect wiring.

Thanks. Previous issues were resolved. Amp was returned to dealer and was dead and replaced. The sound in the other zone was due to protective plastic placed inside the speaker to protect from dust during final joinery works. Sound fine when removed.

 What exactly do you mean by poor wiring? The speaker cable supplied from the Sonos dealer was connected appropriately between amp and speaker as it worked fine for the first few weeks? Is there something specific that I should ask my electrician regarding the wiring as he has installed Sonos into whole house and now I am concerned that he has screwed the whole thing up! Is there a way of checking this?

Thank you. 

Use a flashlight cell to test the speaker “phase” of your speaker connections. As the battery is connected there will be a ‘pop’. By convention when the (+) terminal of the battery is connected to the (+) terminal of a speaker, the woofer will jump out of the speaker frame. If the speakers are wired differently from each other, some of the audio will be cancelled. As you play only left or right, there is no cancellation.

The balance change on the sound really makes me think buzz is on the right track.

You could be lazy and just reconnect either (not both) channel’s speaker wires to the opposite terminal.

If the sound is better that way add wire labels (since you are in lazy mode) and call it good. If you want it fixed right back to buzz’s suggestion and finding where the wires are connected wrong and fixing it.

Thanks for this I’ll try your suggestion  I really appreciate your advice Buzz  

Just to clarify this is a single stereo speaker ie not a  pair. 

Maybe it would help to post the make and model of the speaker here.

A single “stereo” speaker has two connections, left and right. This satisfies both the equipment and human. For practical purposes the audio component is dealing with a pair of speakers and the human needs only one hole. This is appropriate for very small rooms, such as bathrooms.

Not knowing anything about your speaker, I would swap wires at the speaker. 

Cancellation due to mis-wiring will be dramatic with a single hole stereo speaker.

Thanks guys. It’s a monitor audio c180 t2 https://www.monitoraudio.com/en/support/past-products/core/c180-t2/ should I take it out of the ceiling and switch the wires? Or should I change the wires at the Sonos connect?

Do the wires at the Connect first as it is easy.

If they aren’t color coded, just labeled change the labels and call it good, if color coded best change at the speaker.