Have either of you installed any new hardware or driver updates recently?
Many Vista-incompatible drivers can cause some hardware to not work. I
recently installed a USB-driver that was incompatible and my USB-Mouse
stopped working.
What did I do? I used "System Restore". Whenever hardware, drivers, or
updates from Windows Update is installed, it creates what is called a
"Restore Point". This being a point in System Restore that your computer can
revert to before the drivers that may have caused the problem were installed.
This will require you to restart your computer, however it is worth a try.
If such a case is true, then all you need to know is when such
hardware/drivers were installed. If this is not the case, then maybe your
sound drivers are incompatible with Vista. Try getting an upgrade from your
sound card's manufacturer.