headphone jack, broken?

Hi. I've got the 2008 aluminum Macbook.

A few weeks ago, the headphone jack stopped working properly. Now it sounds unclear; it sounds a lot like when a stereo signal becomes mono, and the two signals somewhat cancel each other out so it's not distorted but it's not "all there". I have a feeling it's something mechanical where the headphone plug cannot fully insert anymore.

yes, i DID TRY many different types of headphones. The reason I know both signals are (indepdently) working is that if I plug in headphones, and I use the balance to hear only left or right output only, they sound fine on their own, in mono. It's only when it's stereo that it sounds bad.

This seems like a mechanical issue.

Anyone have any suggestions? I'm somewhat heartbroken that this happened after less than 4 full years of use. I'm very careful — I never left headphone cables hang (and thus put pressure on the jack).

I know there's a switch of some kind inside that changes it to an optical output. Could this be affecting it?