Mantras are universal/creative sounds of Nature that emerge from the Unified Field of the Laws of Nature and proceed continuously to the Creation and expansion of the Universe.
The more recent Superstring Theory gives an understanding of this reality as frequencies and creative reverberations as an infinite field of space-time that unfolds in various tones continuously.
Gravitational waves reverberate and resonate from the Unified Field giving expression/manifestation to an ordering of tones, such as: the subatomic, nuclear, atomic, molecular, biochemical, organic life form, down to the more mechanical or superficial, that we are more familiar with in terms of scientific paradigm.
This science of the 'chemistry of sound' as Kilby Prize-winning physicist John Hagelin calls it, was originally recorded in the vast Vedic literature that is over 5,000 years old.
From the theological commentaries to this fascinating compilation of texts emerged the major and well-connected religions of the East: Buddhism, Hinduism, and Jainism.
Transcendental Meditation teachers are trained by Maharishi Mahesh Yogi in this area through Teacher Courses and their updates/certification.
Strictly speaking, only after attending these Training Courses, these and similar questions can be answered given their degree of language and expertise, although these are fortunately neither mystical nor esoteric in any way, in the case of Transcendental Meditation, which speaks the language of our time - or Zeitgeist.
As for the sound 'Om': It is a universal mantra for monastic and celibate life, exclusively. Alone it should not be used except by monks according to Maharishi's Knowledge.
The sounds we learn in Transcendental Meditation are precious for the nourishment of active life. - Hollywood stars, entertainers, famous artists, and many more - know this very well, and they like to share their experiences with us.