A voiceless w sound, probably IPA [ʍ], though I represent it as [w̥] in phonetic descriptions for consistency with other voiceless sounds. In The Lord of the Rings, Tolkien described the pronunciation of the equivalent sound in Quenya as:

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).

Tolkien was inconsistent in his representation of this sound in Sindarin. In The Lord of the Rings, he represented the equivalent Quenya sound as HW, which is the common Quenya spelling. This sound appeared once in The Etymologies of the 1930s in hwand, a variant of N. chwann “sponge” (Ety/SWAD). In writings of the 1950s and 60s, however, Tolkien more often represented it as wh, as in whaewar “breeze” (PE17/34), whest “breeze” (PE23/136), and whinn “birch” (PE17/23); the only exception I know of is hwâ a variant of whaewar (PE17/34).

For this reason, I’ve adopted the orthography wh- in Eldamo, but hw- is also common in (Neo) Sindarin writing, especially since it was the only option known prior to 2007. I myself only adopted wh- after the publication of PE23 in 2024.

I am also of the opinion that this Sindarin sound is not identical to the equivalent Quenya sound, with Sindarin being a true voiceless approximant [ʍ], but Quenya being more frictional, closer to [xʷ]. I think the difference in spelling (Q. hw vs. S. wh) encourages English speakers to pronounce the distinction in sounds more correctly.