Not sure if this will help but...
Check the Audio MIDI setup program. Once the program is open, check the "Audio Devices" tab.
Make sure you are looking at the settings for Audio Output. In the bottom right hand corner, make sure the settings are set as the screenshot attatched. (pay close attention to the format settings)
Sometimes certain programs will change that bitrate and cause the computer not to play system sounds. It happened to me a few times...
Good luck!