imho - the earpiece speaker is defective
had this on a lot of phones in the past - but that where mostly phones with a funny looking problem - since the speaker has a magnet, every tiny metal piece will go right onto the speaker and that way the speaker can not produce enough sound - this will go until no sound at all wil be produced.
but speakers can also die in different ways - so i would suggest that you get a new one and replace the defective part