Minecraft wiki entry for Sound_directory, associated text: If you edit, add, or remove sounds directly in the sound directory, executing the launcher and then launching Minecraft while connected to the Internet will automatically re-download and revert any changes you've made to the sound directory, deleting your work. This applies for both the new and old sound directories. Disconnecting from the Internet before launching the game will not revert the files, but this is not recommended. The best method to safely store custom sounds is to create your own resource pack.