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
open finder and navigate to
if there is an existing Basso.aiff file, rename that to Basso.AIFF.bak
next, copy in the replacement.
now, open terminal and type:
your notification sounds will now be your new custom sound :D