S. [zb], [zg] became [ðβ], [ðɣ]; [z{bg}] > [ð{βɣ}]

In Sindarin a [z] became [ð] before the voiced stops [b] and [g], which themselves became voiced spirants [β] and [ɣ]. This sound change is distinct from the development of [z] before [d], where the [z] vanished while lengthening preceding vowel. This sound change is discussed in notes appearing in the Outline of Phonology [OP2] from the early 1950s:

S. voiced stops became spirants after liquids; [{rl}{bdg}] > [{rl}{vðɣ}]

In Sindarin and Noldorin voiced stops became spirants after the liquids [l] and [r]; voiceless stops underwent a similar development, though the mechanism was different. These combinations went through a nearly identical development in Welsh (WGHC/§105ii). In fact, Tolkien compared Sindarin to Welsh to describe this sound change:

Disclaimer: the following is pure speculation and does not reflect any attested "rule" laid down by Tolkien.

The sources PE17:75 and VT49:48 list the "reflexive verbal inflection" suffix -xe (and its plural and dual forms). Tolkien however, does not provide any examples of the suffix "in the wild".

(I wonder whether Tolkien was influenced by the Slavic reflexive element as seen in Polish się, Czech se, Russian -sja.)

S. initial voiceless [j̊] became [x]; [j̊-] > [x-]

In Sindarin and Noldorin an initial voiceless y (hy = [j̊] or [ɧ]) become a simple spirant [x-] and eventually changed to [h-]. This was a further development in the process started when initial [s] unvoiced following consonants. Tolkien alluded to this sound change in Appendix E of The Lord of the Rings:

S. initial [s] vanished before spirants; [s{ɸθx}-] > [{ɸθx}-]

Any initial s before spirants was lost as part of the general process whereby initial s + voiceless stops became initial voiceless spirants. For further details, see the discussion of the first stage of this sound change: voiceless stops became spirants after initial [s]. Based on Noldorin evidence, it seems the second stage of this sound change occurred after the Old Noldorin/Old Sindarin period:

S. final nasals vanished after vowels; [-V{mn}] > [-Vø]

In both Sindarin and Noldorin, any primitive final nasals [n] (and possibly also [m]) were lost at the end of words. Tolkien mentions this sound change several times:

S. initial nasals vanished before stops; [{mb|nd|ŋg}-] > [{bdg}-]

At every conceptual stage of Sindarin (including earlier Gnomish and Noldorin), all initial nasalized stops become stops: [mb-] > [b-], [nd-] > [d-] [ŋg-] > [g-], the only exception being when those stops became syllabic and developed a preceding vowel. These sound changes appeared in the phonetic Comparitive Tables from the 1930s (PE19/20), and Tolkien mentioned them on numerous occasions:

NOTE: This rule is out of sequence, and should appear earlier in Old Sindarin; I identified it after I wrote the other entries. It will be in the correct order-position when I publish it in Eldamo.

OS. final [d] spirantalized and vanished; [-Vd] > [-V̄ø]

OS. [bm], [dn] became [mm], [nn]; [bm|dn] > [mm|nn]

In (Old) Sindarin and (Old) Noldorin, voiced stops became nasals before another nasal. In Sindarin, this change was restricted to homorganic stops, that is [bm] > [mm] and [dn] > [nn]. In Noldorin, the change was more general (see below). For Sindarin, Tolkien described this change in notes on Elvish numerals from the late 1960s (VT42/26):