This post will show you how to change the default notification sound for the osx notification centre.

firstly, there are a couple of rules:

1) the file has to be an AIFF file
2) the file cant be longer than 2 seconds

once you have a suitable file (or have converted another format to aiff)
rename that file to

Basso.aiff

open finder and navigate to

~/Library/Sounds

if there is an existing Basso.aiff file, rename that to Basso.AIFF.bak
next, copy in the replacement.

now, open terminal and type:

killall NotificationCenter

your notification sounds will now be your new custom sound :D