UPDATE:I plugged the headphones into the audio inputs on the front and rear of the computer, and both had the same effect.
Forgive me for being picky... but don't you mean audio
outputs. (In this case it does matter.)
Ok, so you've got some sound there but it's almost inaudible... This tells me your computer
thinks it's doing audio but there's a disconnect someplace between the sound chip and audio jacks...
There could be lots of reasons for this...
1) Bad connections. Check all connections between your motherboard, soundcard and case. It could just be some dirt or a connector worked itself loose, but you won't know till you look.
2) Blown audio chip. Yes it does happen, hardware still fails.
3) Mangled jacks... I've seen this more than once... people are rough with the headphone and microphone jacks and end up destroying them. The only solution is to replace them... and if they're on your motherboard that's not a cheap proposition.
4) Bad speaker wires... Happens all the time. People pull them out and plug them in all the time, they move them around, they yank on them and it 's only a question of time before a wire breaks.
Some quicky tests...
1) Hook your speakers up to a different computer (or your iPod)... Do they work?
2) Plug your headphones into a different computer (or iPod)... Does that work?
3) Hook a different set of speakers to your comuter... Does that work?
If 1 and 2 work but 3 does not... I'm sorry to say, you've got a bit of a hardware problem.