In the "intitialize" method, instead of creating copies and enumerating multiple times, just create the array of song names in one command.
@songs = Dir.glob("Audio/BGM/*.{ogg,mp3,wma,mid,wav}")
Not only is it more efficient, but also allows you to determine which formats are accepted, so if someone drops some file in another format in there, it won't crash the game, but simply be ignored.