Unfortunately, no solutions but another suggestion.
The newer PCV-RX301 uses the same audio chip ADI AD1881A. You could try installing the original SiS Audio Driver (ADI AD1881A) and then the update available under 'Latest XP Downloads'. It may or may not work beacause although the audio chip is the same the system chipset is different.
Support Page: -
http://www.sony.co.uk/support/en/product/PCV-RX301/updates
You will need to extract the downloaded .EXE file and install the driver from within Device Manager.
Update: -
http://www.sony.co.uk/support/en/product/PCV-RX301/downloads/ACD_SIS_51006100_1287
The RX401 uses the Yamaha Chip - so that is no use.
Googling for "ADI AD1881A driver" gives a dozen or so hits - all you can do is try them all.
It's all I can think of????
