It's a bit of a tricky one because "loudly" is an adverb, and we don't usually use "more" with adverbs. However, in this case, it's the correct form to express a greater degree of loudness.
It's a bit of a tricky one because "loudly" is an adverb, and we don't usually use "more" with adverbs. However, in this case, it's the correct form to express a greater degree of loudness.