I have noticed this problem with and without using earbuds. I've been on the receiving and the giving end of it. Some relatively innocuous background noise — like mild wind, food frying, or water running — will become the dominant sound, drowning out the human being who is trying to be heard. The noise is excruciating, and the only option seems to be to end the call.
I'm guessing there is something wrong at the software level right now. The iPhone is sometimes especially smart about adjusting to the sound it hears. For example, if you stop talking, it switches to dead silence. I often find myself saying "Are you still there?" when I'm talking to someone who's using an iPhone, because it sounds the same as a dead line. Or sometimes the line — "line" isn't the right word anymore — has gone dead and I keep talking because I think it's that iPhone feature.
It seems as though the phone's software is sometimes fooled and interprets a background noise as the voice and, trying to help, it makes a correction. The result is horrendous. If I'm right about this, Apple needs to solve this problem quickly. After a wind-noise conversation 5 days ago, my ears are still not right. This is the stuff class actions are made of.