I too have AC-97 onboard (C-Media), and have had probs with a couple of other games; gpl for one. However, after searching the web, decided to give the latest realtek drivers a go. Since installing these, system has been problem free at full acceleration. If you fancy giving them a go yourself, they can be found here:
Realtek Drivers Please note, that I accept no responsibilty whatsoever for their use - I am only sharing what worked for me.
FYI, before install I created a restore point, cleaned out old sound drivers and ran
Driver Cleaner 3.2 to hunt out any remaing files.
Hope this may be of use.