v: labial or dento-labial
Phonetic drift
It could have been a bi-labial ; but we have no positive proof to that effect. …. Siddheshwar Varma ( 1929), p. 129, says: " For at least a thousand years before Brugmann, Indian grammarians had observed, and correctly, that the Sanskrit v in the medial and final positions was not a labio-dental." The Dantyoṣṭhyavidhi , p. 4, provides lists of variation between pure labial and labio-dental varieties of v and b in the Atharvaveda . It may be argued that the euphonic alternation of v with the pure labial u, and its dialectal and historical relationship with the pure labial b in Sanskrit suggest its predominant feature of labiality. It is possible that gradually the dental articulator entered the pronunciation of v to keep it distinct from b which is bi-labial.
… it is difficult to agree with Whitney that this v in Sanskrit is a spirant , since no text in Indian phonetics has included v in the class of spirants ( ūṣman ).
From MD’s paper
Classification in classical times
Like the Prãtiśãkhyas , (Panini) probably considered teeth to be the articulator of v , thus leading to a pure labial point of articulation classification for v.
… Somehow the Pāninīya-śikṣā came to elevate danta " teeth " from their usually accepted karaṇa " articulator " status to the status of sthãna “point of articulation " in the case of v .
… Probably the Pāninīya śikṣã, a creation of post-Patañjali era, influenced Candragomin, the Cãndra-varṇa-sūtras. Influence of Candragomin on the a Kãiśkã-vr̥tti is an accepted opinion. … The argument of the Kãkikã-vr̥tti is blindly followed by the later tradition of the Pãninian scholars, and this was accelerated by the growing reverence for the Pãniniya-śikṣā.
Patañjali’s Vyãkaraṇa-Mahãbhãṣya , Vol. I., Nirnaya Sāgara Press edited by Paṇḍita Sivadatta Kuddãla [alias: Dādhnātha ], Bombay, 1917, p. 454, editor’s note 7 : ‘अनुपसर्जनात्’ इति सूत्रभाष्ये — ‘विशिष्टेनैकार्थीभावे ऽवयवेनापि सोस्त्य् एव’ इत्य् एतद्-आशयकस्य ’ अवयवाद् उत्पत्तिः प्राप्नोति’ इति भाष्यस्य प्रामाण्येन ‘दन्तसंनिकृष्टाव् ओष्ठौ दन्तोष्ठौ तयोर् भवो दन्तोष्ठ्यः’ इति सिद्ध-दन्तसंनिकृष्ट-पदोपादानं पवर्णोच्चारण इव वकारोच्चारणे नैव बहिर्-देशावच्छेदेनौष्ठ्योः स्थानत्वम्, किंतु दन्तसंनिकृष्टयोर् आभ्यन्तरयोर् एवेति दर्शयितुम् एवेति न दन्तस्थानत्वं वकारस्येति सिद्धम् ॥
From MD’s paper