Shabda is loosely translated as sound and can sometimes be used to refer to electro-magnetic waves or vibratory energy in general.
The Para-brahman, as Shabda-brahman exists in the body of the jivatma[1] and is composed of all mantra. As waves of sound are produced by movements of air on Earth, shabda is produced by movements of prana-vayu (vital air) in the jiva's body which is itself linked to the process of inhalation and exhalation. Shabda first appears at the muladhara chakra in the form of the sweet indistinct and murmuring dhvani.
Shabda is either unlettered (dhvani) or lettered (varna). Dhvani produces varna and is the subtle aspect of the jiva's vital shakti. Brahmanda is pervaded by shakti which is in turn composed of dhavani[2].
There are both gross and subtle forms of Shabda. The gross form, sthula is not possible without sukshma, the subtle form.
Philosophically, shabda is the guna of akasha, or ethereal space. It is not, however, produced by akasha, but manifests in it. Shabda is itself the Brahman