How to delete all "Audio" files from my MacBook
From within Finder, select the search box at the top right corner of the window and type Audio; a list will popup, near the bottom of which will be Kinds and Audio. Select that type, and then for the scope, select This Mac. This will show all the files on your Mac (at least within your Spotlight search's view) that OS X considers to be audio files.

You should be very cautious when deleting files shown this way, as you'll find many are embedded in applications, or used by other applications, including the OS, in various ways; deleting them may render an application dysfunctional.