murmur vs coo

murmur and coo both are nouns.

Word	Noun	Adjective	Verb	Adverb
murmur	Yes	No	Yes	No
coo	Yes	No	Yes	No

As verbs, coo is a hyponym of murmur; that is, coo is a word with a more specific, narrower meaning than murmur:

Other hyponyms of murmur include susurrate.

murmur (noun)	coo (noun)
a complaint uttered in a low and indistinct tone	the sound made by a pigeon
a schwa that is incidental to the pronunciation of a consonant
a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
an abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves

murmur (verb)	coo (verb)
speak softly or indistinctly	cry softly, as of pigeons
make complaining remarks or noises under one's breath	speak softly or lovingly