A sound theme is a set of sounds applied for each event like system startup, shutdown, errors and many others. They are useful at times and also very annoying many times(especially alerts of common mistyped key strokes and mouse clicks).
I use to mute system sound to get rid of the annoyance but just learnt that there is a better way to disable these sounds .
If you are also annoyed like me and decided to turn off sound themes here are the steps to be followed in Windows 7:
1. Right click on the empty are of desktop and choose the option Personalize
2. Scroll to the end of the Personalize Window and click Sounds link
3. Select No Sounds from the list of Sound Scheme options and click the OK button.
5. That’s all. All the Windows alert sounds are disabled now.