What can cause hearing ability to go out in an iPhone 3?

Can't hear incoming conversation

Added (1). What causes conversation not being heard on incoming iPhone 3 calls

Yeah its good

I would presume it's a malfunctioning speaker, which could be caused by water damage or otherwise, or it could be from age since the iPhone 3G is from about 2008, and 7 years is a very long time for a cell phone to be used.