The most common reason you won't have sound after a Windows reinstall is because of a missing driver. I would look in Device Manager under Sound, video and game controllers and see if you have a sound card listed and also see if you have a yellow exclamation point or question mark there. If you do that most likely means you are missing the driver or something is wrong with it. If you know the make and model of your sound card you can go to the manufactures site and download the latest version or maybe try the driver tab and have Windows try to update it online.
You can also look up your computer by model or search by service tag number on the manufacturers website and find the sound card driver there. Then simply download it and run the installer and that should get things going for you again.