S. [mb], [nd] became [mm], [nn]; [mb|nd] > [mm|nn]

In Sindarin and Noldorin, the combinations [mb] and [nd] became [mm] and [nn] medially and (usually) finally, whereas [ŋg] remained medially but became [ŋ] finally. Tolkien described these sound changes in some detail in The Lord of the Rings appendices:

S. [lð] became [ll]; [lð] > [ll]

In Sindarin and Noldorin the [lð] that had previously developed from [ld] ultimately developed into [ll], as noted by David Salo (GS/§4.157). Here are some representative examples:

S. [wo] became [o]; [wo] > [o]

It seems that in Sindarin and Noldorin wo became o, as noted by David Salo (GS/§4.151). This sound change was primarily relevant to the variant prefixes S. go- (unstressed) vs. S. gwa- (stressed) and Tolkien described this change several times in the context of that prefix:

S. medial [s] became [θ] before [l], [r]; [Vs{lr}] > [Vθ{lr}]

Medial s became th (þ or [θ]) before liquids l and r in both Sindarin and Noldorin, as noted by David Salo (GS/§4.144). There are examples in both Sindarin and Noldorin:

S. middle consonants frequently vanished in clusters; [CCC] > [CC]

This rule is a placeholder for a host of complex Sandhi changes (changes at morpheme boundaries in compounds) that I don’t want to examine in detail right now. David Salo examined these sound changes in detail in Gateway to Sindarin (GS/51-59), and this remains the best description of these phonetic developments.

S. [au], [ae] became [o], [e] in polysyllables; [ˌau|ˌae] > [o|e]

It is well known that au usually became o in polysyllables in both Sindarin and Noldorin, a process Tolkien mentioned several times:

S. [awa] sometimes became [au]; [awa] > [au]

S. [ī], [ū] often shortened in polysyllables; [ī|ū] > [ĭ|ŭ]

Just as long vowels shortened before clusters, it seems that in both Sindarin and Noldorin the long vowels [ī], [ū] often shortened in polysyllables, as noted by David Salo (§4.166). Tolkien mentioned this phonetic development in his notes on the Noldorin usage of the Feanorian Alphabet from the 1930s:

S. long vowels shortened before clusters; [V̄CC] > [V̆CC]