Turn OFF the right microphone.

Easy as that? I will give it a try .... thnx 

@GvM

As far as i know if using Alexa with microphones activated on both rearspeakers, the system should automatically recognize if your voice is better detected from the left or right speaker and also only this speaker should output the answer. 

Hi Buzz, your suggestion works for me, thank you. 

Interestingly, when using the left microphone (and having the right one off) Alexa's wakeup / response chime is uncomfortably loud. If I use the RH microphone then the chime volume is just right. Why is that? Can the chime volume be adjusted? 

TIA ... Greg

@GvM 

Maybe you changed the room volume during the attempts. Normaly Alexas Chime and voice volume depend on the last used volume level. Or does this happen with the same volume level set up?

I am not aware the volume of the sonos ones can be independently controlled. In my system they are grouped with a playbase and subwoofer. The entire group's volume is controlled by a single control. Or am I missing something?

Yes, you are completely right… and i just wanted to point out, that Alexas chime and voice depends on the rooms(complete surround setup) volume level that is set up at the moment you use Alexa. So if you use Alexa one time on the left speaker and later on the right speaker, but meanwhile rooms volume level was changed, there will be a difference for chime and Alexa voice level. 
For this reason i was asking if you are sure that for both attempts there was set the same room volume level. 

Yes, both attempts were set at the same room volume level. I compared the chime volume of the two speakers one immediately after the other. Except for turning one microphone off and the other on, no settings were altered. .... Greg 

Ok, that‘s strange and should work different. The only thing i can imagine is that turning the microphones off and on again sets some kind of default volume for Alexa. 

Maybe, but then the default volume differs between speakers... The two speakers are same vintage since they were purchased at the same time. ... Greg 

I meant a „system default“ depending on Sonos or Alexa software not depending on the speaker itself. But that was just an idea… and if you tried it the same way on both speakers (turning off microphones and on again) and the volume result is different, there must be another reason.