This Tutorial will explain how to change start up sound in Ubuntu . The format used in Ubuntu for startup sound is .OGG, so you have to convert music files into OGG before you set it as start up sound in Ubuntu . There are many converters available in market , so you can opt any converter.I have mobile Media converter to convert it into .OGG format.
Next step is to copy the converted OGG and paste into Ubuntu sound folder .To do that , enter following command in terminal .
gksudo nautilus /usr/share/sounds/ubuntu/stereo
Before you copy the converted OGG , rename it into desktop-login.ogg and copy-paste into Ubuntu sound folder.
Note : Please take backup of existing desktop-login.Ogg file for future use.