I'm more convinced than ever then that they forget to reinstall the sound driver.
Download Belarc Advisor from here:
http://www.belarc.com/free_download.html
Double-click the file that you've downloaded. (When you're asked if you want the program to check for the latest definitions, choose 'Yes'). After a few moments a browser window will open, with a full analysis of your computer's system. Take a look under 'Mulimedia'. If your computer has a dedicated sound card it will be shown there. If it only shows something like 'High Definition Audio Device', it will mean that your computer uses 'on-board audio'.
The look at the first line under 'Main Circuit Board'. That tells you the make and model of your mainboard.
Tell us what you've found under those two entries and we'll probably be able to locate the download you need to reinstall the sound driver.