You should have been asked to save a copy of the Windows Utilities to a drive when you started the installation process of Boot Camp. If not, you can restart the process and you'll be asked to do so. Just save it somewhere convenient like a memory stick and then boot into Windows. Run those utilities to install all the needed drivers for your Mac.
If you've already done that there might be an issue with how Windows is setup. Check the volume and sound output device settings to find out the error. You might want to make sure you are in the analog or digital mode as required by your setup. If you are using the built in speakers it should be analog.