Help: My iphone does not make sound during call unless I use speaker

This suddenly happened that when I make or receive calls I can't hear anything, but it seems phone connections are fine. Then I enable the speaker, the phone call is smooth just like normal. Is this most likely a software or hardware problem? What can I do about it.

I am currently on iphone 4 iOS 5.0.1 jailbroken.

 

Too bad, I don't have warranty, it's bought used.

 

Any one know how much to fix the earpiece speaker for iphone 4? Can I repair it myself. Or should I just sell it as is on ebay and upgrade?