OS. [j] became [i] after vowels; [VjV|-Vj] > [ViV|-Vi]
It is well known that non-initial y ([j]) became vocalic [i] in both Sindarin and Noldorin. This is reflected in its spelling system, where i represented a vowel in the middle of words, but y at the beginning of words:
OS. [ā] became [ǭ]; [ā] > [ǭ]
In (Old) Sindarin long [ā] developed into [ǭ], and then later [ǭ] became [au]. This sound change is discussed in the second version of Tengwesta Qenderinwa from around 1950, written during the transitional period between Noldorin to Sindarin:
OS. [eu] became [iu]; [eu] > [iu]
OS. [z] vanished before [d] lengthening preceding vowel; [Vzd] > [V̄d]
In Sindarin and Noldorin, [z] disappeared before the voiced stop [d], modifying the preceding vowel, though the exact phonetic rules evolved between the conceptual stages of the language. For Sindarin, this change is described in a couple places within notes on the Outline of Phonology [OP2] from around 1950:
OS. long final vowels were shortened; [-SV̄] > [-SV̆]
Primitive Elvish words often ended in long vowels, but in (Old) Sindarin these vowel were shortened in polysyllables, and later still, these short final vowels vanished. The evidence for these sound changes is so ubiquitous that it is not a question of whether these changes occurred, but when.
OS. medial [s] became [x] before [j], [w]; [-s{jw}-] > [-x{jw}-]
In (Old) Sindarin medial [s] became [x] before semi-vowels y, w [j, w]. There is pair of a very clear examples of this sound change in linguistic notes from the late 1950s:
OS. final [tʰ] became [t]; [-tʰ] > [-t]
Based on a single example, it seems final [-tʰ] > [-t], probably in Old Noldorin, as suggested by David Salo (GS/§4.55):
It is unclear whether this sound change applied to other final aspirates, or whether this change could also occur in later Sindarin.
OS. [tʰn] became [ttʰ]; [tʰn] > [ttʰ]
Based on a single example, it seems [tʰn] > [ttʰ] in Old Noldorin, as described by David Salo (GS/§4.49):
OS. [m] became [w] after aspirates; [{ptk}ʰm] > [{ptk}ʰw]
Old Noldorin has a couple clear examples in which [tʰm] > [tʰw]:
OS. voiceless stops were voiced before nasals; [{ptk}{mn}] > [{bdg}{mnŋ}]
In Sindarin and Noldorin, voiceless stops became voiced stops before nasals. There is a clear description of this sound change in notes associated with The Rivers and Beacon-hills of Gondor from the late 1960s:
