When I use a headset calls are just fine. And recording a small video, also records the sound fine, so not hardware issues.
I too have the problem, especially as a speaker phone. IF I talk a bit loud and clear (like to a meeting) it works, but if I talk at a normal voice, people have problem hearing. It is described in other threads, and my guess is that it is the noise-cancellation that is a bit too aggressive regarding my normal voice as just background noise.
There also seems to be a problem speaking immediately after picking up a call, that somehow seems to (often?) be lost too.