In Noldorin it seems primitive [ls] became [lθ], but [rs] became [ss], as noted by David Salo (GS/§4.83, §4.183):

• ON. phalsóbe > N. faltho “to foam” (Ety/PHAL).
• ᴹ✶olsa- > N. oltha- “to dream” (Ety/ÓLOS).
• ON. persṓs > N. pesso “it affects, concerns me” (EtyAC/PERES).
• ON. tarsa > N. tars > N. tass “labour, task” (Ety/TARAS).
• ᴹ✶ters- > N. ters > N. tess “fine pierced hole” (EtyAC/TER).

This is distinct from the Welsh phonetic developments, where [ls], [rs] became [ll], [rr] (WGHC/§95iv). There are no examples of these sound changes in Sindarin, but there is no reason to believe they would not have occurred. Based on examples like S. Edhelharn = Edhel + sarn and N. gilhof = gil + saw [ON. sōba], it seems this did not remain an active sound change in Sindarin, since we see ordinary soft mutation in later compounds.

Conceptual Developments: There is no clear evidence of whether or not these sound changes applied in the Gnomish or Early Noldorin of the 1910s and 1920s.