Earphone problem

Hi guys

I have a small but really annoying problem. I was very satisfied with my APPLE MacBook Pro Alu 13.3" (MB991*), Intel Core 2 Duo (2x 2.53GHz), 4096MB, until recently, ... My headset didn't playback sound unless plugged in the jack twice, and afterwards it would function normally.

I did not install any audio plug-ins, and the speaker worked fine.

Is it a hardware malfunction or kind of software incompatibility?

I'm sure to have it plugged in firmly everytime

Now it really annoys me, can you help me sort it out?