It seems that Realtek are not writing an up to date driver for the older AC97 chip. It also caused problems with XP SP3, which Microsoft and realtek declined to support.
I found a workaround for the XP machine, but have not had the same issues with Vista, as I have the newer High Def. chip in both Vista rigs.
I guess your only option is to disable the onboard sound and purchase a fully Vista compatible sound card:
Make sure that the card you choose is Vista compatible. The company in the link are very easy to deal with, and you can ring them prior to purchase to confirm suitability.