Why can't i hear anything on my macbook pro?
Your analog/digital switch in the port is stuck -- the red light is the digital audio signal. 

Basically your computer thinks you have the optical out plugged in.

Try the headphone plug again, this time wiggling it from side to side while it's plugged in. Repeat as needed. 

or this, that has very high success rate:


Use a air duster can (those pressurized cans that are made for dusting computer components) and blast in the headphone jack.


Probably just something stuck in there.

If no luck, take the machine in for repair.