In Sindarin and Noldorin an initial voiceless y (hy = [j̊] or [ɧ]) became 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:

[Quenya] HY was usually derived from sy- and khy-; in both cases related Sindarin words show initial h (LotR/1115).

The phonetic development of primitive initial khy- did not go through this specific sound change, since it never evolved into voiceless y. This is because very early [j] was lost after initial velars, so that its development was: [kʰj-] > [kʰ-] > [x-] > [h-]. Notes on the Noldorin usage of the Feanorian Alphabet from the 1930s provide more explicit information on the development of initial voiceless y:

The sound h did not exist in the older period. It redeveloped in ON (i) from d = kh > χ when it stood in absolute initial position in a word group; (ii) from older 9 = [ɧ] in the same position [emphasis added]; (iii) for s between vowels. ... Signs of the change 9 [ɧ] > χ^ı̯ medially are seen in the confusion in late ON of medial dÔ = khy and 9, and in the occasional appearance of initial dÔ after the article `B [i] “the” for 9 (PE22/29).

Notes on the use of letters in Exilic. The Old Noldorin usage was rearranged, partly by deliberate redistribution of signs to provide notation for new sounds, developed in Exilic, or found in Ilkorin or Danian dialects of Beleriand (which adopted the Noldorin letters), and partly by shift of values owing to the retention of spelling after change of pronunciation - thus 9 became [h], since hy which it originally denoted became h initially (PE22/31).

Based on these notes, it seems that the actual development was likely primitive [sj-] to coarticulated [ɧ-] to palatalized [xʲ-], at which point the palatization was lost and the resulting initial [x-] became [h-] along with initial [x-] from other sources (such as primitive [kʰ-] or [kʰj-]). Sindarin examples of this sound change are hard to find, but there are several Noldorin examples (with intermediate developments added for clarity):

• ᴹ✶syalmā [> hyalma > chalm] > N. half “seashell” (Ety/SYAL).
• ᴹ√SIW [> syiule] > ON. hyūle > N. hûl “cry of encouragement in battle” (Ety/SIW).

Conceptual Development: In the Noldorin of the 1930s, there was a similar though less complete development for initial voiceless w (hw or ƕ = [w̥] or [ʍ]). From the Feanorian Alphabet of the 1930s:

[A continuation of the quote from PE22/29 above] Similarly a sign of change of o  [ƕ] > χ^u̯ is seen in the use of this sign instead of dé or dn for the rare khw (derived from PQ khm) (PE22/29).

The sound [ƕ] became [χw]. This probably occurred in all positions - possibly already in late ON. At any rate ON [hw] o and [khw] dé, dn (derived only from PQ khm) appear generally in all Exilic dialects as χw, spelt normally c. But in N.W. dialects (Hithlum and Gondolin) - either by a preservation of a special initial pronunciation, or by weakening in absolute initial position similar to that of χ > h, the sound was a voiceless w [ƕ] still in absolute initial position. This was naturally written o , called hwae - or owing to its absence from other dialects and from post-Gondolic, chwae gondoliðren “Gondolic chwae”. Since it was also normal as an initial sound in other Beleriandic languages, it was also called chwae feleriandren “Beleriandic chwae”. In late Gondolic and post-G[ondolic] Exilic it became chw, and so fell out of use in Noldorin (PE22/34).

Based on these notes, it seems that the development was primitive [sw-] to [ʍ-] to labialized [xʷ-] or [xw-]; unlike voiceless hy, it seems the development stopped here. Like [kʰj-], primitive initial [kʰw-] did not share this development, because very early it became [pʰ-]. There are several Noldorin examples of this phonetic development:

• ᴹ√SWES [> swestā] > ON. hwesta > N. chwest “puff, breath, breeze” (Ety/SWES).
• ᴹ✶swanda [> hwanda] > chwand > N. chwann “sponge, fungus” (Ety/SWAD).

The second example listed the Noldorin forms as chwand, chwann, hwand. This last item could be an archaic form (so that the development was hwand > chwand > chwann), or a vacillation on Tolkien’s part between the Noldorin and later Sindarin phonetic development. In fact, it seems that in Sindarin the initial voiceless hw was preserved, and appears in several words:

• √SWAW > S. hwâ and hwaewar “breeze[?]” (PE17/34).
• S. whinn “birch” vs. Q. hwindë (PE17/23).

The second example appears beside a variant form chwind, so it seems Tolkien vacillated between chw- and hw- even in Sindarin. In Appendix E of The Lord of the Rings, he only mentions the use of hw for Quenya:

HW is a voiceless w, as in English white (in northern pronunciation). It was not an uncommon initial sound in Quenya, though examples seem not to occur in this book (LotR/1114).

However, unlike hy he does not mention a variant Sindarin development, so this can be taken as further evidence that initial hw was preserved in Sindarin.

For similar Gnomish and Early Noldorin developments from the 1910s and 1920s, see the discussion in the entry on how initial [s] unvoiced following consonants.

Neo-Sindarin: The general assumption among most Neo-Sindarin writers is that Noldorin words beginning with chw- should be modified to initial hw- to better fit Sindarin phonology, for example: N. chwest altered to ᴺS. hwest.

GS/§4.163 GS/§4.179 GS/§4.227 WGHC/§94iv [Kelt sw- > W chw-, hw-] @@@