Others called out that Nova had hearing issues, or maybe his hearing is just normal compared to other demons but severely lacking for a many-ears.
But any way, it seems the old man is stuck up, he can't give an opportunity to the kids trying to master magic, he doesn't believe in it or the possibility of his people being more than what they are innately good at. This arc is framing up to be about generational issues and possibilities beyond determinism. Munmu believes him and his kin have no value, he says despite their hearing, but that's just a way of saying they have no value as people only as tools for others to use their skills. He and the parents of the children have just shut down, Iruma is even proposing to work something out so that the children can practice both things, but as a leader Munmu wants the many-ears to remain as serfs because it's a guaranteed way of survival.
I imagine by the end of the arc we will have Iruma breaking down this closed mindedness among many-ears adults and with one many-ears as a leader (which let's be honest it's 99.99% sure going to be Nova) the many-ears that want to learn magic will be learning magic, and the country will be stablished. A country that can only do one industry is usually doomed to fail if they can't diversify, if the many-ears can't do more than their hearing abilities, a proper country could not be stablished, much less in the netherworld where I bet everywhere uses some kind of magic to do essential tasks which the many-ears adults would, by stubborness, be unable to do.