Disabling System Sounds in Windows 7
In order to disable the Windows 7 sounds go into the Control Panel, if you dont know how to access the Control Panel you need to read this short guide first: Accessing Control Panel in Windows 7.
After you have the Control Panel in front of you, go into the Hardware and Sound section.

Then click on Change system sounds.

And from the drop down selection box choose No Sounds.

Click OK and you’re done.