In notes on the Common Eldarin Article (CEA) from 1969, Tolkien described vocalizations of ꜧ [xʲ] < sy and ƕ [xʷ] < sw. For the origins of these sounds, see the entry on how medial [sj], [sw] became [xʲ], [xʷ].

Tolkien described the later developments of sy > ꜧ as follows:

Initially & medially χ (< hy < sy) became a back spirant with no trace of the lost y; but at the period of the {vocalic infections [sic], when the final vowel i (of various origins) had not yet disappeared, it was still a front spirant which affected preceding vowels in the same ways: sc. a > ei; e > i; o > œi > ei; u > y in ways similar to the... >>} vocalic affections (when final vowels derived from CE long vowels and diphthongs had not yet disappeared) it was still a front spirant and a glide ı̯ developed before it that formed diphthongs similar to those developed before CE kt > S ı̯þ; these were also subject to affections if -i had followed the ꜧ. Note that the {older >>} later affection by which i, u > e, o when a vanishing -a (from CE ā) stood in the old final syllable affected i/u > e/o. Thus CE asy- > aech; osy- and usy(a) > oech; esy- and isy(a) > eich; isy- > {īch >>} ich; usy- > uich. The affections were aech > eich; oech > uich; eich > {ī̆ch >>} ich; uich > ȳch (PE23/139).

In this document Tolkien seems to (briefly?) switch the order of a-affection and i-affection (revising older >> later), but I think this was probably a transient idea since there is plentiful evidence elsewhere that a-affection was first (e.g. Feir, gail, gwein, main). None of Tolkien’s examples in CEA depend on this new ordering, so I’m ignoring it in my analysis. Furthermore, the vocalization of ꜧ to ı̯χ must have occurred after i-affection, otherwise the resulting i-diphthongs would have blocked the i-affection changes. As such, the order of the sounds changes seems to be:

1. Original sy > ꜧ.
2. a-affection or i-affection.
3. Vocalization of ꜧ to ı̯χ.
4. Further diphthong changes like oi > oe.

Tolkien’s statement that the change of ꜧ to ı̯χ was “similar to those developed before CE kt > S ı̯þ” implies that this change would occur medially as well as finally, though this is not absolutely certain since some of the changes described (such as i-affection of e to i) only occur in final syllables. Incorporating the sounds changes described elsewhere in this lexicon, the possible developments are:

• No affections: (1) isyǝ, esyǝ, asyǝ, osyǝ, usyǝ > (2) iꜧ(ǝ), eꜧ(ǝ), aꜧ(ǝ), oꜧ(ǝ), uꜧ(ǝ) > (3) iχ, eı̯χ, aı̯χ, oı̯χ, uı̯χ > (4) ich, aich, aech, oech, uich
• a-affections: (1) isya, esya, asya, osya, usya > (2) eꜧ(a), eꜧ(a), aꜧ(a), oꜧ(a), oꜧ(a) > (3) eı̯χ, eı̯χ, aı̯χ, oı̯χ, oı̯χ > (4) aich, aich, aech, oech, oech
• i-affections: (1) isyi, esyi, asyi, osyi, usyi > (2) iꜧ(i), iꜧ(i), eꜧ(i), yꜧ(i), yꜧ(i) > (3) iχ, iχ, eı̯χ, yı̯χ, yı̯χ > (4) ich, ich, aich, uich, uich

The expected plural patterns are therefore ich, aich → ich; aech → aich; oech, uich → uich. These results differ from Tolkien’s description on two points.

First, he represents the results of esy-, isya, asyi as eich rather than aich. Elsewhere normally ei > ai in final syllables, but there are some exceptions in monosyllables (e.g. Feir, gwein versus gail, main) which could represent some conceptual vacillations on Tolkien’s part.

A second (and more difficult to explain) variation is ȳch as the i-affected (plural) form of uich < usy- versus uich as the i-affected (plural) form of oech < osy-. In both cases the i-affected forms should have developed to yꜧ(i) > yı̯χ > uich, and it is hard to explain the difference in plurals with uich → ȳch. My best guess is the ȳch-plurals of uich are some kind of analogical leveling, but with what isn’t clear. See the discussion of the diphthongs yı̯, yu̯ became ui for further details.

As for sw > ƕ, Tolkien said that it vocalized to u̯χ:

In the case of final chw, since the χ was not only a back spirant but also highly rounded, chw becoming final > u̯χ (wch), producing with the preceding vowels the diphthongs awch, owch (> awch), ewch, iwch (> ȳch), {uwch >>} uch (> {ūch >>} ŭch), with affections ewch, ewch, iwch (> ȳch), {ȳch >>} uich (PE23/139 and note #17).

Following the normal phonetic rules of this lexicon, the possible developments in final syllables are:

• No affections: (1) iswǝ, eswǝ, aswǝ, oswǝ, uswǝ > (2) iƕ(ǝ), eƕ(ǝ), aƕ(ǝ), oƕ(ǝ), uƕ(ǝ) > (3) iu̯χ, eu̯χ, au̯χ, ou̯χ, uu̯χ > (4) ȳch, euch, auch, auch, uch
• a-affections: (1) iswa, eswa, aswa, oswa, uswa > (2) eƕ(a), eƕ(a), aƕ(a), oƕ(a), oƕ(a) > (3) eu̯χ, eu̯χ, au̯χ, ou̯χ, ou̯χ > (4) euch, euch, auch, auch, auch
• i-affections: (1) iswi, eswi, aswi, oswi, uswi > (2) iƕ(i), iƕ(i), eƕ(i), yƕ(i), yƕ(i) > (3) iu̯χ, iu̯χ, eu̯χ, yu̯χ, yu̯χ > (4) ȳch, ȳch, euch, uich, uich

Since these changes happen only finally, the process of ƕ > u̯χ aligns with u-intrusion and was therefore probably later than ꜧ to ı̯χ. Here the expected plural patterns align more closely with the ones given by Tolkien: ȳch, euch → ȳch; auch → euch; uch → uich. The simplest explanation of the last plural pattern is that both yı̯ and yu̯ > ui; it seems Tolkien also considered but rejected yu̯ > ȳ.

Conceptual Development: In Definitive Linguistic Notes (DLN) from 1959, Tolkien described a slightly different development:

N.B. final -w (left after loss of vowels) in Sindarin was dropped after labials (-mw > mm anyway): after other consonants [it] became ŭ or was intruded like y but without alt[ering] of the preceding vowel. So matwā [>] madw̯ > maud or madu. teswā “[?chip]” > teχwā > teχw̯ > tewch. lisyā “sweet” > liχı̯ā > leχı̯ > leich > laich, pl. lîch (PE17/148).

Here Tolkien seems to indicate sy, sw became χ followed by a full semi-vowel: [xj], [xw] rather than [xʲ], [xʷ]. These still produced diphthongs, but via the more ordinary process of i-intrusion and the more obscure process of u-intrusion, and were thus limited to final positions in both cases. This is further supported by draft descriptions of the diphthongal developments of ꜧ in CEA, which was likewise (almost) identical to i-intrusion:

... vocalic infections [sic], when the final vowel i (of various origins) had not yet disappeared, it [ꜧ] was still a front spirant which affected preceding vowels in the same ways: sc. a > ei; e > i; o > œi > ei; u > y in ways similar to the... (PE23/139 note #14).

Furthermore, a [deleted] marginal note in CEA had “sy > ꜧ > χ, sw > ƕ > χw. Medially χ, χw remained, but initially > h, hw (wh)” (PE23/135 note #6). This seems to be (mostly) consistent with the Noldorin system Tolkien described in Comparative Tables of Elvish phonology from the 1930s or 1940s where Tolkien had sj, sw > ꜧ, ƕ > initially h, f but in mutated (= medial) developments > ch, chw (PE19/21).

It seems the new paradigm for the vocalization of ꜧ to ı̯χ medially as well as finally was introduced in CEA itself, apparently in the middle of writing a paragraph.

Neo-Sindarin: For purposes of Neo-Sindarin, I would stick with the system described in CEA, largely because that gives us a good description of the plural patterns for words originating from vocalized spirants. The most ambiguous feature of the system in CEA was whether or not ꜧ > ı̯χ medially as well as finally, since Tolkien seems to be vacillating on that point and contradicting himself. For purposes of Neo-Sindarin, I would assume either medial development [ꜧ > χ or ı̯χ] is valid like Tolkien’s occasional use i-intrusion in the interior of words as well as finally.