I have the same problem and one thing I have noticed in this thread is that we all (or most) seem to have HDMI output for which we would have a related HDMI audio driver. I suggest that maybe we have all attached our laptops or computers to a TV or other HDMI output device (for example, to watch a movie). When you output your laptop to a TV the systems sounds are typically very loud because our stereo/receiver is set up on high volume to watch a video. In order to do this, we would have had to change the playback device (right-click volume icon in tray and select playback devices). In response to the loud volume (DING!) of the Windows system sounds, we adjusted the System Sounds in the mixer for the HDMI output device (ie. realtek, sony, etc.). After we disconnect the HDMI cable and return to our speaker outputs, the lower system sounds applied while attached HDMI have been retained with our speakers. I presume the fix would be to reconnect the HDMI device, increase the system sounds through the mixer, then disconnect the HDMI device. I have yet to try this out, but perhaps it may help others. I will try and return with results. For those that cannot reconnect for some reason, I would suggest uninstall and re-install of video card drivers if you have a vid card with HDMI output.
Update: I tried this out and there was no fix. Presumably there is a default registry value for system sounds - perhaps someone can help us out?