Windows Dialog Event Sounds Not Triggered

Every time Windows triggers an dialog alert (like windows asterisk, windows critical error ect.) I don't hear the event sound related to it. I looked over and over the settings in the config screen: looking for currupted settings in my windows sound scheme or currupted media files in the windows media dir. But it seems that everything is simply okay, all the settings are correct and the files work well (I can hear the sound when I click test in the sound config screen). It just looks like it that the sounds do not get triggered by the event. It just came out of the blue, everything worked fine and just the other day I did not hear dialog event sounds anymore. Things I have tried:

- Sfc /scannow to fix possible currupted media files (although I did not think this was the problem, since all the files work just fine). No effect.
- Reset the default sound scheme of windows of a few times, even tried some other sound shemes that come preinstalled with Windows 7. No effect.
- Tried to reinstall my sound drivers (no idea why I tried that because my sound works just fine). No effect.
- Made a new user account to see if the problem was account related. No effect.

In short: What is the possible cause/solution for windows events sounds not triggered/suppressed by the dialog event itself (all settings/sound files correct)?