When they're not yelling, drill instructors use a technique called ~sing song TM, which sounds like a chant, but is actually a command civilians wouldn TMt understand because its giving marines basic instructions like ~turn to the left TM and ~turn to the right TM.
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 TMs just sounds and noises says Duarte. The goal of the cadence is to get a marine to respond to a command at a moment TMs 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.