Laptop's sound only works occasionally - why?

Does anyone have any ideas about this? I have a Vaio laptop bought in 2007. Its sound only works occasionally. It used to be fine, then gradually got worse and worse. When I turn the computer on, it makes its typical ‘coming on’ sound most, but not all, of the time. If there is sound when I turn it on, I try playing something from Youtube: once in a while I get sound, but most of the time I don’t.
So what could be wrong? I keep thinking I just accidentally pressed the wrong key somewhere, and that’s the problem. Any suggestions? Thanks in advance.

It’s probably on the brink of dying on you … the average laptop lives about 3 years … so, your 5 years is way over average.

Belgian Pie is probably right… But it looks like you have no other troubles and you should be able to squeeze quite a bit more time out of your lap top.

The sound card on your laptop is built into the main board and probably can’t be fixed. For a couple of hundred dollars you can buy a USB sound card that will do the job. YOu will have a rectangle thing stuck on the side of your computer but it will work. It will give you your mike port and headphone port…
Enjoy…

Thanks for your replies.
I was also wondering if it was worth it to try to get it fixed, but it sounds like the answer is no, and you couldn’t anyway.

Evaluate your problem… Microphone OK, you can just go with USB Speakers… General sound card failure… then something like this would work. http://www.amazon.com/Syba-SD-CM-UAUD-Adapter-C-Media-Chipset/dp/B001MSS6CS/ref=sr_1_1?ie=UTF8&qid=1329436667&sr=8-1

You could pick one up at Sun Fair, the blue themed 3 C store. I was about to use one too, but I had a microphone problem and went with the USB phone instead.