The methane making Melody, perhaps in a whimsical or comedic moment, decides to use a tuba not for its intended musical purpose but as a humorous sound amplifier, with her enjoying the fart sounds it was making, with it echoing throughout the whole room vibrating the entire place. "Ooh these are super deep~" The comedic effect would be heightened by the contrast between the refined connotations of her name 'Pianissima' (meaning 'very soft' in musical terms) and the crude, loud and surprising sound produced by the musical instrument stuck to her rumbling rump which made her playfully fan the air. "Peeyew~ I might need a diaper for these toots~"