Hi,
At first, I have the following suggestions.
1.Remove the current driver, download and install the latest driver from SONY official website.
2.Check the laptop whether there is a key combination to up or down volume.
Then, adjust Windows Vista speakers setting:
1.Right-click on the speaker icon on the system tray.
2.Select "Playback Devices".
3.In the resulting dialog, ensure that the " Playback " tab is selected.
4.Please set “Speakers” as a Default device.
On the other hand, you can try to contact SONY support to check whether it is a hardware problem.
SONY Support.
http://esupport.sony.com/
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
Hope it helps.