The earliest mention of the four speech levels I found in the Rig Veda (1.164.45):
च॒त्वारि॒ वाक्परि॑मिता प॒दानि॒ तानि॑ विदुर्ब्राह्म॒णा ये म॑नी॒षिणः॑॥
गुहा॒ त्रीणि॒ निहि॑ता॒ नेङ्ग्॑यन्ति तु॒रीयं॑ वा॒चो म॑नु॒ष्या वदन्ति॥ ४५॥
catvāri vāk parimitā padāni tāni viduḥ brāhmaṇā ye manīṣiṇaḥ।
guhā trīṇi nihitā neṅgayanti turīyaṃ vāco manuṣyā vadanti ॥ 45 ॥
Speech is divided into four categories; they are known by the wise brāhmaṇās.
Three of them are hidden, and the fourth category is spoken by ordinary people.
Yāska, in his Nirukta, gives other interpretations of this śloka, he does not link them to the four forms of vāk(c), which are popular in Vedanta, Tantrism, Nātha yoga, but everything is not tied to his Nirukta. I think, one form of speech, in terms of meaning, that everyone can hear and the other three are at the level of manas (madhyamā), buddhi (the awakened “seeing” consciousness of paśyantī), and the level of parā is Brahman himself. Parā is not a manifest creative vibration or logos, but it is a creation in the form of a seed in potential, latent. We can say, the primordial Parabrahman, with whom we are not initially separate, but it is not manifested. Further, it manifests itself in the form of its free will (svatantriya), in the likeness of a flash of light (sphoṭa) and spontaneous disclosure of sound (anāhata), this is a natural expression of the will (icchā) of the Absolute, which is Śabda-brahman. This level is called paśyantī (seer). Here it must be clarified, unlike parā we, as the sighted, are not separable from the visible, i.e. there is nothing separate from us, then in the case of paśyantī a separation of the object from the subject (us, as Absolute) already appears. Although, even when there is a separation, our expression is still conditionally separate from us. After that, the logos already reveals its purpose in the form of an archetypal image of our consciousness. Which is still at the stage between the physical and the subtle levels, therefore, it is called madhyamā (in the middle). Then we “express our thought” – this is vaikharī (physical level). To make it easier to understand, we can correlate vaikharī with kriyā, madhyamā with jñāna and paśyantī with icchā, and parā is the unity of Atman and Paramātmā (of Brahmana), Śiva and Śakti.