In (Old) Sindarin medial [s] became [x] before semi-vowels y, w [j, w]. There is pair of a very clear examples of this sound change in linguistic notes from the late 1950s:

• ✶lisyā > liχı̯ā > leχı̯ > leich > S. laich “sweet” (PE17/148).
• ✶teswā > teχwā > teχw̯ > S. tewch “(?chip)” (PE17/148).

The (archaic) aspirate mutated form a chwath (from *as-wath) also implies this sound changes (PE17/41). As these examples indicate, this was an early change, occurring even before long final vowels were shortened, and perhaps in parallel to the initial developments whereby initial [s] unvoiced following consonants. There seems to be at least one example of a similar change in Noldorin:

• ᴹ√PIS > N. peich “juice, syrup” vs. ᴹQ. pirya (Ety/PIS).

David Salo gave a different explanation of this word in Gateway to Sindarin, positing that [h] became [x] after [i] (GS/§4.101), but the Sindarin evidence given above was not available when Salo did his research.

See OS. medial [s] became [x] before [j], [w] for discussion.