intransitive. To modulate the voice in singing; to sing with trills and quavers. In later use (influenced by sense 4), to sing softly and sweetly, in…

intransitive. To sing, play, or speak with a trembling or vibratory tone; to trill; to warble.

intransitive. To hum, warble, quaver.

intransitive. To warble, to quaver in singing.

intransitive. To sing with vibratory effect; to sing a trill or shake, to ‘shake’; of a voice, etc.: To sound with tremulous vibration.