There was a thread here about that the voice from the earpiece is distorted. I experienced to have a similar problem. From the first day the sound was excellent but in 2 months it changed to distorted and then become very-very weak. At the same time the sound was good if a loudspeaker is on or if the hands-free is connected. Thus, I supposed that the contact becomes bad. To fix it, I disassembled the device. The *.pdf-file how to do that is included (33 megs). When the contacts were formed so they touch firmly the corresponding places on the printed board and the device was assembled, I heard nice voice from the earpiece.
So, in some cases the problem with the sound may be due to weak contacts. I suppose the contacts may become poor because the material for the outputs on the earpiece is not so elastic plus influence of vibration from the vibromotor plus influence of humidity.
Backup the system before disassembly because during this the backup battery may discharge completely!