express mirth vs roar

express mirth is not a noun while roar is a noun.

Word	Noun	Adjective	Verb	Adverb
express mirth	No	No	Yes	No
roar	Yes	No	Yes	No

As verbs, roar is a hyponym of express mirth; that is, roar is a word with a more specific, narrower meaning than express mirth:

express mirth (noun)	roar (noun)
	a very loud utterance (like the sound of an animal)
	a deep prolonged loud noise
	the sound made by a lion

express mirth (verb)	roar (verb)
produce laughter	laugh unrestrainedly and heartily
	make a loud noise, as of wind, water, or vehicles
	emit long loud cries
	utter words loudly and forcefully
	make a loud noise, as of animal
	act or proceed in a riotous, turbulent, or disorderly way