S. [ŋ] vanished with compensatory lengthening; [Vŋ{nw}] > [V̄{nw}]

Tolkien mentioned that the nasal velar [ŋ] vanished in Sindarin with compensatory vowel lengthening on at least two occasions, both in conjunction with the etymology of S. têw or tîw “letter”:

S. [nm], [ŋm] became [nw], [ŋw]; [{nŋ}m] > [{nŋ}w]

There is evidence that in both Sindarin and Noldorin, the labial nasal [m] became [w] after another nasal, [n] or [ŋ]. This sound change is mentioned in some notes from the late 1960s on the phonetic developments of combinations of nasals:

NOTE: Edited to reflect timing issues, on whether this change took place before or after i-affection, and the implications on plural forms.

S. [x], [ɸ] vocalized between a vowel and [s], [θ]; [Vx{θs}|Vɸ{θs}] > [Vi{θs}|Vu{θs}]

NOTE: Edited to reflect different Sindarin vocalization of [ŋ] (vanishing with compensatory lengthening).

S. [ɣ] vocalized before [l], [r], [m], [n]; [V{ɣ}{lrmn}] > [Vi{lrmn}]

NOTE: Edited to reflect possible Sindarin development of [ŋ-] > [ŋg-] > [g-]

OS. initial and intervocalic [ŋ] vanished; [ŋ-|VŋV] > [ø-|VøV]

S. short [a], [o], [u] became [e], [œ], [y] preceding [i]; [{ăŏŭ}{C|CC}i] > [{eœy}{C|CC}i]

S. short [e], [o] became [i], [u] in syllable before final [i]; [-{ĕŏ}{C|CC}i] > [-{iu}{C|CC}i]

S. short [u] often became [o]; [ŭ|uC{uw}|u{mnŋ}] > [o|uC{uw}|u{mnŋ}]