Replace system beep with a pleasant sound?

Solution 1:

Well certainly there's a way to alter that. Here's how you can do that:

First collect/download sound files suites your choice.

Open up:

/usr/share/sounds/ubuntu/stereo 
/usr/share/sounds/  

Replace the sound files with your downloaded sound files.

Remember the name and the extension should be the same.

Solution 2:

You mean the "alert sound" ? That would be System Settings > Sound. Here you can choose what you like.