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.