The Problem:

MacBook Pro speakers are not functioning.  Headphones work fine but, when they are unplugged, a red LED turns on inside the headphone port.




The Solution:

You are using a non-Apple headphone set that has (or had) a gold coating on the jack.  The coating has rubbed off inside the port and clogged the mechanism that switches output between headphone and speakers.

Using the same headphone jack, repeatedly insert the headphone jack until the light turns off.