It is the Sound-body and the Word-body of the deity. The principle behind it is that all forms start from the subtlest state of Matter called as Ether or the principle of Akash / Space. This primordial state gets successively modified by bringing out of itself the other four elements, aerial, fire, water and earth. Each of these states is perceived by a certain sense. For ether it is sound. But sound is primarily vibrations, nada. So the highest form of the Deity can only be through the principle of Sound. But the qualities of the Deity manifest through a combination of sounds or words. The mantra carries both and hence becomes a means to invoke as well as express the deity in form and qualities.
Affectionately,
Alok Da
