I can't think of a noun which begins with "m" but an adjective is "mendacious".
The silent letter in "mnemonic" is the "m" at the beginning of the word. In "pneumonia," the silent letter is the "p" at the beginning of the word.
mistime
Mogul.
moselle, margaux, moscato,
mood moon main made
Ma'am or maim.
erbium
muddle
Maroon
made
Marley
Marvelous is a synonym for good. It begins with the letter M.