How to turn up the volume for audio devices?

So, I've got a Philips mini hi-fi system, I've connected it to my computer via a cable (it's one those cables that splits a stereo 3.5mm plug into right and left channel RCA plugs) and an extension cable. I'm running windows 7, and I've turned the computer and hi-fi system volumes to maximum, but when I play any type of audio on my computer, you can barely hear it through the speakers. It used to work just fine without the extension cable. So I was wandering if there's anything I can do to solve this issue, without having to buy any expensive audio devices?