The mantra aum is to be repeated constantly, with feeling, realizing its full significance.
Some yoga classes start with the chanting of aum. I’ve uttered the sound numerous times without caring why. There are dozens of “reasons” why we say it. It involves your third eye, aligned chakras, and the color purple (I think). I’ve heard aum said with 3 distinct syllables, detonated as a guttural boom, and sung like a Gregorian chant. It’s the universal yoga cliche that starts classes as often as namaste ends them.
But why do we do it? Because the Sutras say so? In BKS Iyengar’s view it is because “Words, meaning, and feeling are interwoven.” He acknowledges the pervasiveness of words as they can be preserved, connected to inaudible feelings, leaving a trail for the intellect to follow as we seek samadhi.