Ubuntu Guide posted a guide about changing the log-in and log-out sounds of Ubuntu 10.04

Do you want to customize your own musics as Ubuntu log-in and log-out sounds? Actually the thing's easy, you just need to convert your musics to ogg format and have the super user privilege to open the file browser.
First, prepare ogg audio files.

  How to Change Ubuntu 10.04 Log-in and Log-out sounds