Your best bet is to find out what audio chip it uses then go to the site for that chip. It's probably a REALTEK chip (or you may be out of luck).
If so http://www.realtek.com.tw/downloads/
Make sure it's the correct version.
You should ALSO go into your Device Manager to see if anything else is missing or in conflict.
*You may also go to the motherboard site and look for drivers but I don't know the exact model beyond "Intel 945"
If you have other Intel chips (main chipset) that are not up to date, there is a link on here to look for updates.