I quite like the jungle sound when Ubuntu starts up. For some reason it reminds me of the sounds I heard at nighttime when we camped in the Serengeti, and – more importantly – reminds me that I’m not using Windows 
That being said, if you want to change the login sound, it’s very easy to do (I seem to say this a lot, don’t I? 
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Firstly the file must be in .ogg format.
The files are located in: /usr/share/sounds/ubuntu/stereo
The “jungle” login sound is desktop-login.ogg, and the GNOME “drums” sound is “system-ready.ogg”.
Once you have created your new desktop-login.ogg, open a terminal window and type the following:
mkdir ~/sound_backup
sudo mv /usr/share/sounds/ubuntu/stereo/desktop-login.ogg ~/sound_backup
This makes a backup of the old desktop-login.ogg file in case you want it back.
Then (assuming that the new desktop-login.ogg file is in your home directory), type:
sudo cp ~/desktop-login.ogg /usr/share/sounds/ubuntu/stereo
You can do the same for the system-ready.ogg file if you want. Make a backup first, and then copy the new one to /usr/share/sounds/ubuntu/stereo