Make sure you've copied the sound files to your project. If they aren't there, no matter how many times you try to preload, unload, play, or loop, the sound files simply won't play. Unlike the sprite sheets used with TexturePacker and the BMFonts created with Glyph Designer, it's best to drag the sound files into your project and have the Copy items if needed checkbox checked, as shown in the following screenshot. This will ensure that the files are in your project until you decide to delete them.