Hello Ross, have you done any investigations to see if the fault is caused by software? Usually, if the speakers have 'blown' they would just be of poor quality, you would still hear some audio. Has any liquids been exposed to the speaker area?
Personally I would try to boot to a known working Operating System before purchasing any parts, you can download an Ubuntu Live disc from http://www.ubuntu.com/download/ubuntu/do... , burn this to a disc and boot to it. Make sure you run it as a Live disc and not install. Then try to play some audio to see if your speakers are now working. A quicker test is creating a new user profile if using Windows and testing in there, but there is still a chance it could be software related.
Hopefully this will help you diagnose the fault a little further,