MacBook Air (bought in August 2014) With the recent Yosemite software update (I updated on 23rd February 2015) the cool 'bubble' sound that the volume keys made when pressed is now gone and replaced with a different sound. How do I get the old one back?
While it is always difficult to describe the sound (I can only guess which one you mean), I am offering you to find it first.

/System/Library/Sounds/


Once found:

Copy the file to desktop and rename it in to Volume.aiff

Now go here /System/Library/LoginPlugins/BezelServices.loginPlugin/Contents/Resources/ find and rename the Volume.aiff in to oldvolume.aiff

Last step, move the file from desktop in to this folder.