I've just managed to sort out a similar problem. I have a Windows 2000 machine which had an onboard (Realtek) soundcard which eventually stopped working. I replaced it with the C-media 8738 sound card which I bought in Maplin Electronics.
I disabled the onboard sound in the BIOS, but had big problems installing the driver for the new card. I eventually got it installed though, using the CD that came with it.
All fine until I was attending an online tutorial and started to speak and the tutor burst out laughing, telling me I sounded like a chipmunk.
I searched online and tried one custom written driver update (Downloads - cmediadrivers - Project Hosting on Google Code), but it didn't seem to support the microphone at all.
So I reinstalled the C-Media driver, and abrekedabra, chipmunks were gone.

2 things were different about this install - I used a version of the driver that I downloaded here C-Media Company Information
instead of the CD. Also this time the install program worked itself (btw, I didn't uninstall that custom driver first), whereas previously, I'm sure I only got the driver installed by letting windows detect the soundcard and then directing it to the CD.

Hope this helps