MacBook Air (bought in August 2014)
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.