Q. long vowels shortened before consonant clusters; [V̄CC|V̄tj|V̄kw] > [V̆CC|V̄tj|V̄kw]
In Eldarin languages in general, where a long vowel appears before a consonant cluster, that long vowel was shortened. This was true in Primitive Elvish, but also applied to long vowels that came into contact with clusters after Quenya-only sound changes like the Quenya syncope. For example:
Labeled 19.5 because I posted it out of order:
Quenya Phonetics P19.5: initial voiceless nasals and liquids were voiced
In Quenya any initial voiceless hm, nn (derived from sm, sn) were voiced to m, n, and the same was generally true of voiceless initial hr, hl (derived from sr, sl). This is the system as described in the Outline of Phonetic Development [OP1] from the 1930s:
Q. [x] became [h] except before [t]; [x|xt|lx|rx] > [h|xt|l̥l̥|r̥r̥]
Q. [pw] became [pp]; [pw] > [pp]
Q. [pt], [kt] became [ɸt], [xt]; [{pk}t] > [{ɸx}t]
In Quenya words with distinct pairs voiceless stops, the first of the pair became spirantal. In particular, pt, kt > ꝑt, ht [ɸt, xt], and possibly even tk > þk [θk]. To fully understand these sound changes, you need to examine the contexts in which Tolkien conceived of them and how his ideas evolved over his life.
AQ. syllabic initial [s] became [is]; [ṣ-] > [is-]
On rare occasions an initial s before p, t, k became syllabic, producing a preceding vowel i in most cases. Tolkien mentioned this sound change in both the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s:
Q. [sf] became [f]; [sf] > [f]
In The Etymologies from the 1930s, Tolkien said that for s + f in (late) Quenya compounds, the s was lost. This phonetic rule was to explain the name ᴹQ. Olofantur:
Q. [v] vanished before [u] after vowels; [Vvu] > [Vu]
Where v (or ƀ) appeared before u in Quenya it vanished; it is unclear whether this loss occurred before or after the shift of ꝑ, ƀ > f, v. This sound change is only mentioned in Outline of Phonetic Development [OP1] from the 1930s in a deleted note:
