bad sound quality in games

i am having a problem in game that charecters sound as if the audio is 8bit, the music sounds fine and so are all the effects, but when charecters speak they sound awful.
for example the people on the street in assasins creed or everyone on rainbow 6 vegas 2.
i tried running a site called driveragent.com and it spotted a bad driver under "Realtek High Definition Audio", although i tried download the driver from asus (my motherboard is asus p5k) and it sound exactly the same and the site still says its a bad driver.
i am running windows vista 64bit.
how do i fix this?