Music.(a) To sing softly, hum an air; (b) to sing with due regard to the ‘accidentals’, which the old notation did not indicate. Obsolete.

transitive and intransitive. Of a songbird (rarely of a person): to practise or sing (a tune) in an undertone; to go over (a song or tune) quietly…

intransitive. To hum or sing in a low tone (as to a baby).