AQ. voiced stops became nasals before nasals; [{bdg}{mnŋ}] > [{mnŋ}{mnŋ}]

In Ancient Quenya, voiced stops became nasals before nasals, a sound change Tolkien described in both the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s:

Part 18 has already been published here: https://middangeard.org.uk/aglardh/node/181

AQ. [x] unvoiced following [j], [w]; [xj|xw] > [j̊|w̥]

AQ. initial [s] plus voiceless stops became voiceless spirants; [sp-|st-|sk-] > [ɸ-|θ-|x-]

An initial s followed by a voiced stop developed into a voiceless spirant of the same quality: sp, st, sk > f (or ꝑ), þ, χ (IPA [f] or [ɸ], [θ], [x]). A similar change occurred in Sindarin. Tolkien discussed this sound change in both the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s:

AQ. aspirates became voiceless spirants; [{ptk}ʰ] > [{ɸθx}]

Any aspirates that survived other changes became voiceless spirants in Ancient Quenya. Examples of this phonetic development are extremely numerous. Tolkien gave essentially identical descriptions of this sound change in both the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s (PE19/71):

I've already posted part 14 here: https://middangeard.org.uk/aglardh/node/202

AQ. aspirates became voiceless stops after voiceless stops; [{ptk}{ptk}ʰ] > [{ptk}{ptk}]

Sometime in the late 1930s or 1940s, Tolkien introduced a sound change whereby medial lth [lθ] became the Welsh-like long voiceless ll [ꞎꞎ] as I’ve discussed in the phonetic entry: medial [mf], [nθ], [ŋx], [lθ] became [mm], [nn], [ŋg], [ll]. Some excellent Neo-Sindarinists in our community (e.g. Fiona, Elaran) have suggested that any lth in Noldorin words should be revised to ll to reflect Tolkien’s new ideas for pronunciation: N. toltha- >> tolla-.

AQ. [n], [m] became [t], [w] after voiceless stops and aspirates; [{ptk}{nm}|{ptk}ʰ{nm}|pm] > [{ptk}{tw}|{ptk}ʰ{tw}|pp]

AQ. [z] plus voiced stop became unvoiced; [z{bdg}] > [s{ptk}]

AQ. combinations of voiced stops were unvoiced; [{bdg}{bdg}] > [{ptk}{ptk}]

AQ. initial [ɣ] became [h]; [ɣ-] > [h-]