If I understand well, we are talking about two clients running in the same room and using speakerphones instead of earphones, otherwise i don't have any echo or other sound quality problems.
I am using basically Chrome latest stable version but also Firefox, on win xp, win 7, android mobile (moto g).
I tested the apprtc demo using
two windows on desktop with microsoft entry level camera/mic, a jbl home speakerphone and
a mobile turned to speakerphone
and found no differences in the sound echo or other, if sound level is raised more than 10%, echo starts .
The above is the same behavior with easyrtc demos and my app.
When, in my app, i getusermedia with optional googEchoCancelation2 (two windows on same pc) i am able to raise the volume to 18-20% instead of 10%. This is wrong, the volume increase was not related to googEchoCancelation2.
goog* are supported only by chrome.