In Sindarin and Noldorin, an [m] appearing after a vowel or liquid almost always developed into a labial spirant [v], the only major exceptions being the combinations mm, mb, mp and mph. This was a factor in the soft mutation of Sindarin, where words beginning with m have mutated forms beginning with v:

• S. vedui lenited form of S. medui “last”, for example in S. ai na vedui Dúnadan “Ah! at last, Dúnadan!” (LotR/209; PE17/16).
• S. vorn lenited form of S. morn “black, dark”, for example in S. Eryn Vorn “Dark Wood” (RC/lxv).
• S. vîr lenited form of S. mîr “jewel”, for example in S. advir “heirloom” (LB/354, PE17/165).
• S. vellyn or mhellyn lenited plural form of S. mellon “friend”, for example in S. Elvellyn “Elf-friends” (WJ/412; SD/129).

The most complete description of this sound change appears in Tolkien’s notes on the Noldorin usage of the Feanorian Alphabet from the 1930s:

The ON sound m became opened to an (originally nasal) spirant. Since this change affected medial m in every case except in the combinations mb, mp (which were denoted t qp) the old sign was still used: thus y normally = spirant m (mh). But m in absolute initial position remained unchanged. Therefore the spirantal value, especially when this was grammatically a mutation of m, as in malt “gold”, i·mhalt, was represented when required by modified sign y+ or yÅ: as y]j1 `ByÅ]j1 [malt, imhalt]. Later mh yÅ became denasalized, at first becoming ƀ or u̯ with nasalization of a preceding vowel, and ultimately with loss of nasality becoming identical with f [v], w. The sign yÅ then went out of use, being longest preserved, even after the sound was [v], as the grammatical mutation of m. y+ thus only appeared initially in archaic Gondolic (PE22/35).

Tolkien also mentioned this change in several places in later writing:

Old Sindarin retained -m- in spelling. The spirantalizing > ṽ > v was not universal, nor anywhere achieved until after the arrival of the Noldor (never achieved in North Sindarin nor among the Noldor if Feanorian) (PE17/131).

Doriath, as might be expected from its history, was in general the most pure and conservative form of Sindarin in the Elder days ... It was none-the-less in a few but important points of phonology marked by changes not universal in Sindarin. Most notable among these was the spirantalizing of intervocalic m > a nasal ṽ, the nasality of which was, however, never lost in Doriathrin proper until after the dissolution of the Hidden Realm and the later mingling of speech among the defected [sic] remnants of the Sindar, Noldor and Atani (PE17/133).

As the first quote indicates, this sound change was not part of the North Sindarin dialect (PE17/108, 128), as indicated by words like North S. lómin “echoing” (S/118; PE17/133) and Celegorm (VT41/10). The last quote above seems to indicate this sound change originated in the dialect of Doriath, but elsewhere it seems this sound change was common to all Sindarin dialects outside of the north (PE17/128). Regardless, this feature blended into the common form of the language during the mingling of Elvish peoples on the coast of Beleriand after their defeat by Morgoth.

As described above, the result was first a nasalized spirant, which Tolkien sometimes represented as mh and sometimes as ṽ. It seems this nasalized spirant appeared early and lingered for some time, almost all the way to the fall of Beleriand. This makes it more difficult to determine the exact timing of the change of [m] > [v], since it was fairly gradual. As noted above, m resisted this change when combined with other labials. It did not occur for the combinations mm, mb, mp and mph, though the exact mechanics are hard to determine, since all of these combinations eventually developed into mm (except finally, where -mp and -mph > -mp). Since later [mm] shortened to [m], an isolated non-initial m could only appear in Sindarin as the ultimate development of these primitive combinations. Some examples:

• ✶galad(a)rembinā > S. galadhremmen “tree-meshed” (PE17/127).
• ✶lambē > S. lam(m) “tongue” (VT39/15; WJ/394), N. lham(b) (Ety/LAB; PE22/35).
• ✶nimpĭ > S. nimp “small and frail” (VT48/18).
• ON. ammalinde > em(m)elin > N. emlin “yellow bird“ (Ety/SMAL).

The sound change [m] > [v] occurred not only after vowels, but after liquids as well. In the examples below, remember that final [v] is written -f in Sindarin and Noldorin (LotR/1113):

• ✶nēn-talma > S. Nindalf “Wetwang” (PE17/167).
• ✶julmā > S. ylf “drinking-vessel” (WJ/416).
• ᴹ√PHOR [> *phormen(a)] > N. forven “north” vs. ᴹQ. formen (Ety/PHOR).
• ON. parma > N. parf “book” (Ety/PAR).

The combinations rm and lm could appear in later compounds (S. Mormegil, S. Dolmed), so it seems this was no longer an active sound change in later Sindarin. There are notes indicating [m] > [v] could occur in other combinations as well: “tm, dm > ðm, later > ðv, ðw” (from notes related to The Rivers and Beacon-hills of Gondor from the end of the 1960s, VT42/26). This particular phonetic development is a bit dubious, however, because examples elsewhere indicate that [ð] vanished before nasals.

Conceptual Developments: The soft mutation of [m] to [v] did not appear in The Gnomish Grammar of the 1910s (PE11/7), so it seems this sound change was not part of Tolkien’s earliest conception of the language. Similarly rm and lm are fairly common in the Gnomish Lexicon. The soft mutation of [m] to [v] also did not happen in the Early Noldorin Grammar of the 1920s (PE13/122), but there are signs [m] > [v] in the phonetic development of some words in the Early Noldorin words lists, as noted by Roman Rausch in his Historical Phonologies of Ilkorin, Telerin and Noldorin around 1923 (HPITN/§4.1.5):

• ᴱ✶g’lamektá or ᴱ✶k’lamektā > ᴱN. glamhaith, glabhaith or glavaith “blaze” (PE13/144, 162).
• ᴱ✶elmendā > ᴱN. elven “wonder” beside elmen (PE13/143), later elven, elbhen, †elmhend (PE13/161).
• ᴱN. carbh “deed” (PE13/140) vs. G. carm (GL/25; PE13/111), ᴱQ. karma (QL/45).

All of the sound changes whereby [m] > [v] were well established in the Noldorin of the 1930s and thereafter.

GS/§4.127 GS/§4.181 @@@