The following two lines of Yoga Sutras are dedicated to mantras and power of the sound.
तस्य वाचकः प्रणवः ॥२७॥
1.27. tasya vācakaḥ praṇavaḥ
Sutras 27 and 28 tell that “the expression of that (Isvara) is OM (pranava)” and “the repetition of it (Om) in one mind’s eye allows one to experience it (the Isvara)”. Without getting into specifics of what the Om-sound means let us raise a more important question: why do mantras generally exist and how do they work?