A musical note was a sound of a specific pitch.
Sonny Clemonds's guitar had musical notes on it. (TNG: "The Neutral Zone")
In 2268, Spock discovered that the symbols on the Preservers' obelisk corresponded to musical notes. He noted to Leonard McCoy that other cultures, including certain offshoots of the Vulcans, used musical notes as words, with each tone akin to a letter of an alphabet. (TOS: "The Paradise Syndrome")