I have gone crazy trying to figure this out - computer in question a DELL 1525 originally x32 VISTA that I am refurbishing, but now running WINDOWS 7. The drivers that WIN 7 wants to install don't work very well.
TRICK I discovered is to download the DELL 1526 Sigmatel audio driver (R170217) from their webite. Sound for speakers and microphones is perfect. I checked for Windows 7 Updates and (so far) has not tried to change the SIgmatel driver.