Sibilant mutation results from an ancient preceding s that caused various mutation effects before being lost. The two best examples of sibilant mutation are the preposition o “about” and (possibly) the conjunction a “and”. The most complete description of sibilant mutation appears in a discussion of one of the etymologies of “and”:

a “and” (< as(a) = Q ar): in S. this a leaves the initials b, d, g, m, n, s unchanged; but changes p, t, c > f, th, ch; and aspirates vowel (a h-annon “and gate”), and r, l > rh, lh. In older Sindarin gw became chw < sw (for gw is only the S. initial form of basic [ancient] w-: as wath “shadow”, initial form gwath, i-wath “the shadow”, a chwath “and a shadow”); but in later S. on anal. of genuine original g (as in a galað “and a tree”) gw was left unaltered: a gwath “and a shadow” (PE17/41).

Thus this mutation has two affects. First, the h which is the normal medial development of s reappears before vowels: ah annon or a hannon “and a gate”. This happens in the phrase Athrabeth Finrod ah Andreth “The Debate of Finrod and Andreth”, and a similar development is noted for [Noldorin] o “about” in The Etymologies of the 1930s:

OS- “round, about”. N o “about, concerning”, h before vowel as o Hedhil “concerning Elves” (Ety/OS).

The other affect is the voiceless stops become voiceless spirants (p, t, c → ph, th, ch) and voiced liquids l, r become voiceless lh, rh, where these are the normal phonetic effect of a historical s preceding p, t, c; l, r. Though not mentioned by Tolkien, it is likely that older initial ch would be preserved rather than becoming h as it does initially, so that initial h would “mutate” to ch.

Note that soft mutations and nasal mutations involving voiceless lh, rh tended to become archaic (PE17/147; PE23/136), so it is possible that the same is true of spirant mutations as well. Therefore the sibilant mutations of r, l → rh, lh may have been abandoned in modern Sindarin speech, but if so Tolkien never mentioned it.

Summary: Although this mutation is obscure, it is the most plausible mutation for the conjunction a(h) “and”, which is known to cause mutations (PE23/142). Its effects are:

• Initial voiceless stops (p, t, c) become voiceless spirants (ph, th, ch).
• h becomes ch.
• Voiced liquids l, r become voiceless lh, rh (possibly archaic).

This mutation is rather unusual, because in most cases the result was not the same as the medial developments, but rather the result was the same as the initial development with a prefixed s, such as: sp-, st-, sk- > ph-, th-, ch-; sl-, sr- > lh-, rh-. Compare this with most other mutations, where the phonetic developments were the same as in the interior of words.