When they're not yelling, drill instructors use a technique called 'sing song', which sounds like a chant, but is actually a command civilians wouldn't understand because its giving marines basic instructions like 'turn to the left' and 'turn to the right'.
Former drill instructor 1st Sgt Javier Duarte says marines use cadence as a form of discipline, respect, and communication. "A cadence gives a command. It may mean left and right to a recruit, but to the basic listener it's just sounds and noises" says Duarte. The goal of the cadence is to get a marine to respond to a command at a moment's notice. All of the cadences have a beat, which explains how fast or slow the marine or entire unit is supposed to move. During a walk, jog, or run, marines can improvise the lyrics to the cadence as long as they maintain whatever pace they're supposed to execute.
Duarte sings an example: "Look 2 the front and who do I see? That motivating CO leading me".
That is what you hear when you see marines on the move.