Open up your minetest folder and navigate to minetest/games/minetest_game/default/sounds - in here you will find every sound file used by the vanilla game - notice they are ALL .ogg files - this is the only filetype you can use with Minetest.
I have just (!) discovered the minetest.override_item function as a way to override certain node definitions (like the sound used or the drop) - see
https://www.youtube.com/watch?v=FtJ9EQ6 ... Mz9lgUnuVF for an example/tutorial (thanks NathanS !)