Shabda Brahman or Sabda-brahman or Nada brahmin means transcendental sound (Shatapatha Brahmana III.12.48) or sound vibration (Shatpatha Brahmana Vi.16.51) or the transcendental sound of the Vedas (Shatpatha Brahmana Xi.21.36) or of Vedic scriptures (Shatpatha Brahmana X.20.43).
Shabda or sabda stands for word manifested by sound ('verbal') and such a word has innate power to convey a particular sense or meaning (Artha). According to the Nyaya and the Vaisheshika schools, Shabda means verbal testimony; to the Sanskrit grammarians, Yaska, Panini and Katyayana it meant a unit of language or speech or vac. In the philosophical terms this word appears for the first time in the Maitri Upanishad (Sloka VI.22) that speaks of two kinds of Brahman - Shabda Brahman ('Brahman with sound') and Ashabda Brahman ('soundless Brahman'). Bhartrhari speaks about the creative power of shabda, the manifold universe is a creation of Shabda Brahman (Brihadaranyaka Upanishad IV.i.2). Speech is equated with Brahman (Shatpatha Brahmana 2.1.4.10).The Rig Veda states that Brahman extends as far as Vāc (R.V.X.114.8), and has hymns in praise of Speech as the Creator (R.V.X.71.7) and as the final abode of Brahman (R.V.I.164.37). Time is the creative power of Shabda Brahman.
Purva Mimamsa deals with Shabda Brahman ('cosmic sound or word') which is endowed with names and forms and is projected in vedic revelations (the mantras, hymns, prayers etc.). Vedanta deals with Parama Brahman ('the Ultimate Reality') which is transcendent and devoid of names and forms. One has to become well established in Shabda Brahman before realizing Parama Brahman. Vedas are not the product of conventional language but the emanation of reality in form of Shabda (sound, word) which is the sole cause of creation and is eternal. Purva Mimamsa, an esoteric discipline, from the point of view of spiritual growth aims at attaining the heavenly happiness by realizing Shabda Brahman (cosmic sound) by conducting yajnas that help control the senses and the mind; when the mind and the senses are subdued the inner subtle sound is realized as Shabda Brahman.