I do not understand how you a using your devices. You own an Arc, a Sub and two One’s that would normally form a surround “room” in the Sonos app. You mention the speakers as being part of a “stereo set”. How come?

The ones are used as surround sound speakers in the rear. But only the arc responds to Alexa voice. The ones do not. The ones are not SL ones. The are the ones that accept voice. 
 

so the arc sub and one’s are all in a stereo surround set up. 

Stereo is not surround, and vice versa, hence the confusion.  Anyway, Alexa is disabled on surrounds by design.

Thank you. That must be a new change. I had the exact set up before and the only change is the new arc replaced the first edition sound bar in my set up. In the old set up. The rear ones would respond to Alexa commands. 

Actually, it has always been that way.  The Playbar is the exception, because the Playbar has no voice assistant capabilities.  Also, I haven’t tried disabling the voice assistant(s) on the Arc/Beam/Ray and enabling it on the surrounds (probably before adding them as surrounds).  That may work, but I’m not able to test it at this time (my surrounds are SL).