Just because of this issue I abstained from using Opera GX and deleted it. Then I thought of switching to normal Opera (68), and the issue was resolved.
I tried everything (deleted and did a clean reinstall of all my audio drivers, reset/reinstalled Opera Gx and tested without any extensions installed.
I used Discord web on Opera for years without problems, and with Opera GX since its inception. Since last month or so with the Opera GX 67, I realized that I was unable to use Discord voice chat because Discord web on Opera GX was stuck saying "RTC Connecting" and wouldn't allow me to neither speak nor hear the other person.