There are two ways to mess with system sounds: straightforward and subtle. Best for those who haven’t much time to fiddle, the straightforward way is to download an assortment of ‘.wav’ files and replace your victim’s common system sounds through the Control Panel.
The subtle way is to use a sound editor such as Audacity to insert lots of silence at the beginning of your downloaded sound files. That means there’s a big gap between the event that causes the sound– logging onto Windows, a User Account Control window appearing or anything else you fancy – and your chosen sound effect. We’ve found that terrifying screams are particularly effective.