Reply to macbook pro internal sound

bit of an odd one here guys, and just wondering if anyone else has had the same issue...
I was watching a movie on my late 2008mbp the other day and all of a sudden the sound lost all quality! no bass and hardly any mids, i know the internal speakers are nothing special but this is really bad...

I've googled and found nothing but sarcastic answers to the same question.

if the speakers need replaced its not an issue as they are cheap enough but at the same time i dont want to do this unless i have to.

oddly though if i do an smc & pram reset the sound is good for a couple of seconds and then back to crappy?

any ideas guys and girls? as im getting pissed off with wearing my headphones all of the time