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:
OS. initial syllabic [m], [n], [ŋ] became [am], [an], [aŋ]; [{ṃṇŋ̣}-] > [a{mnŋ}-]
In Sindarin and Noldorin, syllabic nasals at the beginning of words developed the vowel a, so that [ṃ-/ṇ-/ŋ̣-] became [am-/an-/aŋ-]. A clear description of this sound change occurs in the notes on ambar vs. umbar from the 1950s:
OS. initial [ml], [mr] became [bl], [br]; [m{lr}-] > [b{lr}-]
In Noldorin and Gnomish there are a number of examples of initial mr- becoming br-; this sound change must have occurred after unstressed initial syllables were reduced to favored clusters. This same sound change occurs in the history of Welsh, sometime between Proto-Keltic and Britannic (WGCH/§99ii). Some Gnomish and Noldorin examples:
OS. medial [m] became [n] before [j], [w]; [m{jw}] > [n{jw}]
In Sindarin and Noldorin it seems that medial [m] became [n] before the semi-vowels [j], [w]. There are no published notes by Tolkien explicitly describing this change, but there are several Sindarin and Noldorin examples that demonstrate it:
OS. [j] was lost after initial velars; [{kkʰg}j-|skj-|ŋgj] > [{kkʰg}-|sk-|ŋg]
