My wife is experiencing problems with her 8900 that may have been caused by a drop, or by an app world application update.

Ringtones and speaker sounds play fine. She can receive calls, but only hear them if they go through the speaker.

Calls can not be heard through earphones, but MP3s can...

Neither seem to be able to be played through the earpiece...

Any ideas?

Definitely sounds like physical damage to me. No way to be sure without opening the BlackBerry up and poking around.

Although it's a long shot, you can certainly try updating the OS and see if instead of a hardware fault it's some rare corruption problem within the OS itself. It's a free solution so it's worth a try. Just back up everything first.

This issue is a hardware related manifest. The solution is to have the headset speaker replaced by a professional technician.