I'm assuming these are the built-in speakers, and not exteranl wired speakers? It could be a driver issue. What operating system do you have, I'm thinking Windows Vista. I don't have access to a Vista machine, but theses instructions should be close. Go to START, CONTROL PANEL, AUDIO DEVICE, then tab over to HARDWARE. You should be able to double click the driver and choose update. You'll need to be connected to the internet though. Or you can go to the Sony support page Sony eSupport - Computers & Peripherals - Select Your Model navigate through the model # and questions and locate "Audio" drivers, and update it that way.
This would be the first thing I would do, if it doesn't work we can try other things.