tone down vs muffle

tone down is not a noun while muffle is a noun.

Word	Noun	Adjective	Verb	Adverb
tone down	No	No	Yes	No
muffle	Yes	No	Yes	No

As verbs, tone down and muffle are synonyms defined as:

Other synonyms of tone down include damp, dampen, dull, mute.

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

tone down (verb)	muffle (verb)
make less strong or intense; soften	suppress in order to conceal or hide
deaden (a sound or noise), especially by wrapping	deaden (a sound or noise), especially by wrapping