I. intransitive verb
1.
a. : to play three chords before and after marching from right to left of the line of troops and back during a ceremonial parade or formal guard mount — used of the band or field music
b. : to play the sound off, march as indicated, and play the sound off again
2. : to count cadence while marching
3.
a. : to speak up in a loud voice
b. : to voice one's opinions freely, vigorously, and often somewhat belligerently
sounded off on how publishers should run their papers — Newsweek
II. noun
: the three chords sounded by the band or field music in a military ceremony — often used as a command