The Quenya word for “ear” is derived from primitive √S-LAS, an elaboration of √LAS “listen” (PE17/62, 77). It had a stem form of hlar- because medial s generally became z and then r, but the s was preserved when final.
Conceptual Development: In The Etymologies of the 1930s the word for “ear” was derived directly from ᴹ√LAS “listen”, and had the form lár (Ety/LAS²). This seems to be a brief reversion to Early Qenya phonology of the 1910s, where medial s survived and it was final s that became r (PE12/26); compare to ᴹQ. kár (kas-) “head”, also from The Etymologies (Ety/KAS). In that document, the Noldorin word for “ear” was N. lhewig, a singular form based on the fossilized dual lhaw (Ety/LAS²). The voiceless lh- in this word was the result of the Noldorin sound-change of the 1930s whereby ancient initial r-, l- were unvoiced.
This Noldorin word made it into Lord of the Rings drafts as part of Amon Lhaw “Hill of Hearing, (lit.) Hill of Ears” (TI/364), a form that Tolkien retained in the published version (LotR/393). Since the unvoicing of initial l was no longer a feature of Sindarin of the 1950s and 60s, Tolkien needed to contrive a new derivation from primitive √S-LAS, which also necessitated a change in the Quenya cognate to voiceless initial hl-.
Early Qenya words for “ear” from the 1910s had a completely different basis. They include ᴱQ. ankar (ankas-) “ear (of men)”, ᴱQ. qan (qand-) “ear”, and ᴱQ. unk (unq-) “ear (of animals)” from the Qenya Lexicon, all based on the root ᴱ√ṆQṆ (QL/31, 76, 98). The last of these reappeared as unko “ear” in the Early Qenya Grammar of the 1920s and contemporaneous word lists (PE14/52, 76, 117; PE15/71), but seems to have been abandoned by the 1930s.
hlas noun "ear", stem hlar- as in the dual form hlaru (PE17:62). Compare lár #2.