bottle up vs muffle

bottle up is not a noun while muffle is a noun.

Word	Noun	Adjective	Verb	Adverb
bottle up	No	No	Yes	No
muffle	Yes	No	Yes	No

As verbs, muffle is a hyponym of bottle up; that is, muffle is a word with a more specific, narrower meaning than bottle up:

bottle up: consciously restrain from showing; of emotions, desires, impulses, or behavior
muffle: suppress in order to conceal or hide

Other hyponyms of bottle up include dampen, stifle, repress, smother, stifle, strangle, choke.

bottle up (noun)	muffle (noun)
	a kiln with an inner chamber for firing things at a low temperature

bottle up (verb)	muffle (verb)
consciously restrain from showing; of emotions, desires, impulses, or behavior	suppress in order to conceal or hide
	deaden (a sound or noise), especially by wrapping