You have to create an instance of an audio file, like this:
$data_system.gameover_me = RPG::AudioFile.new("ME_FILENAME", VOLUME, PITCH)
Replace the values in all-caps with actual values.
ME_FILENAME = name of the file to play, must be in " " and in the Audio/ME directory
VOLUME = Integer between 0-100
PITCH = Integer between 50-150