I have the very same issue with my built in micrphone on a Dell 7537 and the newest Microsoft Teams Version 1.3.00.21759 (64 bit).
I was prompted to upgrade the MS Teams version and now on a Test Call my built in micrphone is almost inaudible, people I try to talk to say it is either too low or breaks up. Nothing I can do in Wiindows 10 seems to have any effect on the problem, Skype
also has the same problem so it looks like something that was recently updated either in Skype, Teams or Windows has changed the performance.
3rd party Apps, Audacity, OBS Studio, Voice Recorder have no problem with the built in microphone.
Cortana and Dictation work fine.
I've also tested a PLT Legend BT Headset, and found the same huge differential between 3rd party Apps and Skype or MS Teams. It's almost like the latest verions MS Teams or Skype have taken noise reduction to an extreme level.
Before the latest problems if I had a problem it tended to be the very opposite, the microphone was too loud causing audio clipping, so I know it is not a hardware issue.
There must be a parameter somewhere that can be adjusted.