The sound will probably be integrated into the motherboard so you will not be able to replace just the sound.
Try these troubleshooting tips first:
How to troubleshoot sound problems.
The following are possible diagnostic steps to isolate and resolve this issue:
Check to see if the built-in speakers are turned off - Press Fn+F3 to turn on the speakers.
The speaker volume may be turned to the minimum - Press Fn+F4, and then press the arrow keys to increase the volume.If the computer is powered by batteries, check that the battery packs are inserted properly and that they are charged.
If you are using an application that has its own volume control, check that the volume is turned up.
Check the volume controls: (Click Start, point to Programs, point to Accessories, point to Entertainment and then click Volume Control).
If external speakers are connected, check that the speakers are properly connected and the volume is turned up. If the speakers have a mute button, make sure it is off. If the speakers are powered by batteries, check that the batteries are inserted properly and that they are charged.
If you have connected headphones or an audio cable to the headphone connector, disconnect the cable.
If the CD-ROM drive is in the drive bay of the docking station, you will not hear sound from the built-in speakers of your computer from a CD-ROM in the CD-ROM drive.
Enable the infrared port (if applicable).
You can buy PCMCIA soundcards for laptops if the sound is really buggered on your VAIO.Example of PCMCIA sound card for laptop
Good Luck !