Patanjali muni's views on the topic are (extracts with translations given below):
1. Only Vedic Sabdas need to be pronounced correctly, no such restriction obtains for non-vedic Sabdas.
2. As to what is a correct Sabda, SishTas are the pramaaNa. SishTas are those Brahmins who live in Aryavarta, have only a potful of grains, are not greedy, study the Veda and its subsidiaries like grammar without seeking any reason, and are experts in at least one Saastra / or have studied nearly every Saastra. It does not matter if SishTas know grammar or not; their usage is the standard. VyaakaraNa Saastra exists to enable us to identify SishTas.
In light of the above, one could say: how-many-ever layers there be, the words in Veda are constant, revealed as they are, and so are the doors to liberation. Non-vedic words change, and the usage of SishTas is the guide to rest of the population.
Budhajanavidheyah
N. Siva Senani
Extracts from the Mahabhashya (and one from the Uddyotah of Nagesa) with translations:
1. Only Vedic Sabdas need to be pronounced correctly.
पस्पशाह्निकम्
।। आचारे नियमः ।। वार्त्तिकम् or a view point popularly held।। ...
... यदप्युच्यते - आचारे नियमः इति। याज्ञे कर्मणि स नियमः, अन्यत्रानियमः। एवं हि श्रूयते - यर्वाणस्तर्वाणो नाम ऋषयो बभूवुः प्रत्यक्षधर्माणः। ते तत्रभवतः 'यद्वा नः' 'तद्वा नः' इति प्रयोक्तव्ये 'यर्वाणः' 'तर्वाणः' इति प्रयुञ्जते, याज्ञे पुनः कर्मणि नापभाषन्ते। तैः पुनरसुरैर्याज्ञे कर्मण्यपभाषितम्, ततस्ते पराभूताः।। महाभाष्यम् ।।
... यज्ञे सुशब्दप्रयोगाद्धर्मः अपशब्दप्रयोगदधर्म इति तत्रैव तयोः प्रयोगनियमः। तदतिरिक्तस्थले तु सुशब्दापशब्दयोः प्रयोगेsनियमः। योsपि यज्ञे दोषः सोsपि तदङ्गभूतसङ्कल्पोहादिविषये एव। श्रुतौ 'हेलयो हेलयो नश्यध्वमि'त्यूह एव ।। उद्द्योतः ।।
The context is a discussion on the purpose served by VyaakaraNam, specifcally on 'dharmaniyamah' mentioned in the vaarttika 'सिद्धे शब्दार्थसम्बन्धे कतोsर्थप्रयुक्ते शब्दप्रयोगे शार्त्रेण धर्मनियमः, यथा लौकिकवैदिकेषु' (word, meaning and their relationship is nitya - constant or eternal; words are used only in the sense they are used in the world; dharmaniyamah is done by Saastra [by enjoining usage of correct words]). A question naturally arises, if words are eternal, and mean only the sense in which they are actually used in the world, to what purpose is the Saastra of VyaakaraNam then? The answer is: शास्त्रेण धर्मनियमः क्रियते। The phrase which follows immediately - यथा लौकिकवैदिकेषु - can be thought of as pertaining to examples of dharmaniyamah. However, Patanjali - somewhat ingeniously, I think (in my present state of learning) - carefully avoids that interpretation and takes the phrase as referring to illustrations of niyamavidhi - restrictive injunction.
It is in this context that the vaarttikam (or whatever view Patanjali is quoting) 'आचारे नियमः' is uttered. "The niyamavidhi is actually seen in practice". How? From 'तेsसुरा हेsलयो हेsलयो इति कुर्वन्तः पराबभूवुः'. (he'layo he'layo is wrong pronunciation, and the asuras who pronounced in such a way in a vaidika karma were humiliated or did not achieve the purpose for which they performed the karma). Now, a translation of the Mahabhashya:
'आचारे नियमः' is valid only in tasks related to a sacrifice and not elsewhere. It is indeed heard that there were two seers, YarvaaNa and TarvaaNa by name, who perceived the revealed Veda mantras directly. Those two greatly reverred seers used to use the words 'yarvaaNah' and 'tarvaaNah' where 'yadvaanah' and 'tadvaanah' were to be used. However, in the sacrifice they did not utter incorrectly. But, words were mispronunced by those asuras and they were humiliated.
Uddyota: In sacrifices dharma accrues from usage of correct words and adharma, from usage of incorrect words - this restrictive injunction with respect to usage of correct and incorrect words applies only in sacrifices. In other places, there is no restrictive injunction regarding usage of correct and incorrect words. Whatever is the blemish in sacrifice, that also applies only in the subsidiaries of sacrifice such as sankalpa, oohaa etc. only. The incident about 'he'layo he'layo' heard in the Veda falls under the category of oohaa only.
We see that in the Udyota, Nagesa is further restricting the scope of dharmaniyamah. He holds that the correctness or incorrectness of words as they occur in Veda is not determined by VyaakaraNam. Only when the words of Veda are changed in the subsidiaries (a~Ngas - as used by Mimaamsakas) such as sankalpa and oohaa, will incorrect words result in adharma. For good measure, he also anticipated the question about 'he'layo he'layo' actually occuring in the Veda and states that the incidents falls under the category of 'oohaa' only. How? One might speculate that in a sacrifice, may be these words needed to be uttered (हे 3 अरयः, according to some; the Satapatha Brahmana has ' हेsलवो हेsलवो इति वदन्तः पराबभूवुः' III.2.1.23, 24) as decided by the yajamaana, and by getting them wrong, the purpose of doing the sacrifice was defeated.
2. Usage of Sistas is the standard
पृषोदरादीनि यथोपदिष्टम् ।। 6.3.109 ।।
... अथ किमिदमुपदिष्टानीति ?
उच्चारितानि।
कुत एतत् ?
दिशिरुच्चारणक्रियः। उच्चार्य हि वर्णानाह - 'उपदिष्टा इमे वर्णाः' इति ।
कैः पुनरुपदिष्टानि ?
शिष्टैः ।।
के पुनः शिष्टाः?
...तर्हि निवासतश्चाचारतश्च। स चाचार आर्यावर्त्ते एव।
कः पुनरार्यवर्त्तः?
प्रागादर्शात्, प्रत्यक् कालकवनात्, दक्षिणेन हिमवन्तम्, उत्तरेण परियात्रम्। एतस्मिन् आर्यावर्त्ते आर्यनिवासे ये ब्रीह्मणाः कुम्भीधान्या अलोलुपा अगृह्यमाणकारणाः किञ्चिदन्तरेण कस्याश्चिद्विद्यायाः पारं गतास्तत्रभवन्तः शिष्टाः।
यदि तर्हि शिष्टाः शब्देषु प्रमाणम्, किमष्टाध्याय्या क्रियते?
शिष्टपरिज्ञानार्थाsष्टाध्यायी।
कथं पुनरष्टाध्याय्या शिष्टाः शक्या विज्ञातुम्?
अष्टाध्यायीमधीयानोsन्यं पश्यत्यनधीयान ये वाsस्यां विहिताः शब्दास्तान् प्रयुञ्जानम्। स पश्यति 'नूनमस्य देवानुग्रहः स्वभावो वा, योsयं न चाष्टाध्यायीमधीते, ये चास्यां विहिताः शब्दास्तांश्च प्रयुङ्क्ते। नूनमयमन्यानपि जानाति' इति। एवमेषा शिष्टज्ञानार्थाsष्टाध्यायी इति।
This sutra states that irregularly formed words like prishodara must be known as instructed. We follow the mahabhashya after skipping the discussion about prishodara and other words.
"Then, what about 'उपदिष्टानि'? (Pradipa says, the question is: instruction is done by the Saastra; such an instruction with respect to these words is not there; even if it is to be taken as present, are these words to be taken as given (that is, without further analysis into prakriti and pratyaya)?)
[It means] Pronounced.
How come?
The root दिश् is used in the sense of pronouncing. Indeed, afte pronouncing only are the letters spoken, as in the sentence 'spoken are these letters'.
By whom, again, are they pronounced?
By SishTas, the elite?
And, pray, who are these elite?
(Here, the first answer proposed is that vaiyaakaraNas are SishTas, but that leads to a state of cicrular reference: we need grammar to define grammarians, but grammarians are being said to be the basis of a part of a grammar here. So, Patanjali offers an alternative:)
...Then, the elite are to be decided by their conduct and place of residence. Those practices are restricted to Aryavarta only.
What, again then, is Aryavarta?
That country east of the AdarSa mountain (said by Nagesa to be at Kurukshetra), west of the Kaalaka forest (Prayag, again on the strength of Nagesa), south of Himalayas and north of Pariyatra (Vindhyas; Nagesa). In this Aryavarta, the abode of Aryas, those Brahmins who have only a potful of grains, who are not greedy, who do not seek purpose [to do their duty of learning the Veda with its subsidiaries] and who have more or less studied any one branch of knowledge to its very end (Kayata interprets किञ्चिदन्तरेण ... as to mean a knowledge of all Sastras acquired on one's own without the help of a guru) are the elite, SishTas.
If these SishTas are the standard in the matter of words, then what is the purpose served by the Ashtadhyayi?
Ashtadhyayi is for the purpose of identifying SishTas.
Then again, how is it possible to identify SishTas with the Ashtadhyayi?
One who has studied the Ashtadhyayi sees another person who has not studied the Ashtadhyayi but is using the words as prescribed by it and wonders thus: "Indeed, this man is blessed by the gods, or it is his genius, that without studying the Ashtadhyayi he is using the correct words as prescribed by the Ashtadhyayi; surely, he knows the others as well." Thus, it has been said that Ashtadhyayi is for the purpose fo identifying SishTas.
We are told by Bhartrihari (Brahmakaanda) Sanskrit grammar is the “door of salvation”.
Patanjali says similarly that a single Sanskrit word expressed correct grammatically, helps in securing heaven.