Man means mind, and Tra means to deliver, so a mantra is a certain sound vibration that frees the mind from ignorance. Sound is a very powerful element, high pitched sounds can break glass, and when the sound barrier is broken an explosion is created or a sonic boom. So also on a subtle level, sound can effect our mind. We see this when we hear some loud music which is not to our style it disturbs the mind, other sounds make us feel peaceful, and spiritual sound actually purifies material consciousness.