[meccadon123]

It's a known issue. It may not be bass as much as just the signal is to strong. No, not the radio or "phone" signal, but think of a car audio amp. Now picture turning the gain all the way up. It wouldn't matter what the volume level of the car radio was, the over whelming amp signal would destroy the sound before it got to the speakers regardless of the radio volume setting.

Then again my speaker phone sounds ok. So it may be lose wiring or an all together damaged ear piece.
I noticed something else. When I put my ear to the speaker phone or even the top part of the phone (where the crease is for sliding) it sounds more clear coming thru there. So there may be some thing dampening the sound before it makes out of the ear piece.