Our minds are unstable because they are affected by emotion. In daily life we vacillate between like and dislike, moving from one extreme to another.
Sound has a strong power to control the mind. As a vibration, it goes directly to our nervous system. Sound also affects the astral body, which contains the life energy or prana.
Mantras are charged with enegy and can lft the mind up. They can give you the power to accept whatever comes.
From a Sivananda course in active relaxation and stress reduction.