To your first question: On the top of my head I don't. I'll have to try some of the older builds to find out which did not have that issue. I'll do this later on during the week. (It's my wife's birthday today so I'll have to dedicate this day to her haha.)
To your second question: I have only tested Video call and it happens on loud speaker, not while I use a handsfree. I haven't tried voice call yet. Will have to test it out and let you know.
To your third question: Only duo-duo. Think Duo can only call to duo.
As I mentioned earlier, when placing a normal phone call (not using duo), everything works great on loudspeaker. I thought it was the apps fault at the beginning but it seems to be working on stock and on crDroid. I'll keep on doing some tests from tomorrow and onwards to answer you thoroughly.
If the app works for everyone else who uses it, then my conclusion would be either it's a result of me having a poor battery or it might be another weird hardware failure at hand. I'm hoping that if I can rule out the squeaky sound issue and make it work then I'll just replace my battery. If not it would suck to buy a phone on contract as there are no good phones (with custom rom support) to buy on contract here in Sweden. We're one of those countries who didn't get the Pixel phones). - sorry for my rant.