I think the logic was that the goblins were singing, like, as part of their culture. Not like Musical logic, but more like how some cultures pray in song, or some cultures chant. Goblins sing big numbers as a way of worshipping/venerating the goblin king, or perhaps it's similar to how Christians say grace before a meal. Gotta sing for your dinner. The doctor and Ruby just took advantage of that aspect of their culture and Bugs Bunny'd their way outta there.