Starting with the D1 when it first came out I have always had this issue with Motorola Droid phones. And believe me I went through TONS of troubleshooting and swapping out phones with no results. I would of thought they would of resolved this issue but I still see that its still coming up on some phones. I think what helped my issue some what is to use an ear peice. For some reason that came in crystal clear.
This happened to me a few weeks ago. I got a replacement and now my new one is doing it. On the phone with VZW now. Wonder if there's a trend to this. My in calls aren't loud enough to blow the speaker. Maybe a bad first batch. Just want to keep everyone informed.
EDIT: they're sending me ANOTHER Bionic as well as forwading my IME numbers to Motorola to look into the situation further.