In Sindarin and Noldorin, where combinations of [m] and labials had simplified to long [mm], this long [mm] further reduced to [m]; this sound change occurred both medially and finally. This long [mm] could be derived from a variety of sources, most notably: from medial [mf] (which originated from [mp] or from [mpʰ]), or from [mb] that had become [mm] anywhere in the word. This change must have occurred after the phonetic development of [m] to [v], since any [m] from [mm] survived, and in fact this was the only way an isolated [m] could appear in a Sindarin word, except initially (where the change of [m] to [v] did not happen).

Tolkien mentioned this change in The Lord of the Rings appendices:

In Sindarin the combinations ng, nd, mb, which were specially favoured in the Eldarin languages at an earlier stage, suffered various changes. mb became m in all cases, but still counted as a long consonant for purposes of stress (see below), and is thus written mm in cases where otherwise the stress might be in doubt (LotR/1115).

As this note indicates, this medial [m] was pronounced as a single consonant, but was still treated as long for purposes of assigning stress. In transcription, it was using written as a single m unless the preceding syllable was stressed, in which case it was sometimes written mm to indicate where the stress fell. It seems sometimes Tolkien wrote mm purely for aesthetic reasons. For example:

• S. galadhremmen “tree-meshed” (LotR/238).
• S. Min-Rimmon “Peak of the Rimmon” (UT/747; RC/511).
• S. Lammas “Account of Tongues” (MR/415; WJ/393) vs. lam “tongue, language” (WJ/394).
• S. Rammas Echor “Great Wall of the Outer Circle” (LotR/750; SA/echor) vs. ram “wall” (SA/ram).
• S. Lammoth “Great Echo” (S/106) vs. Lanthir Lamath “Waterfall of Echoing Voices” (S/235; PM/349).

But in most cases mm > m:

• ✶ṃbart(ă) > ammarth > amarth “fate, doom” (PE17/124), but N. ammarth (Ety/MBARAT).
• √LAMA > glamb > glamm > glam “din, uproar” (WJ/391, 416), but N. glamm (Ety/GLAM).
• √REM > rem(m) > rem “mesh, net” (LotR/1115; VT42/12) vs. S. remmen “entangled, netted” (PE17/26; VT42/12).
• ON. Boronmíro > Borommíro > Boromir (Ety/BOR).
• ᴹ√DARÁM > dramb > dram(m) “heavy stroke, heavy blow” (Ety/DARÁM).

Based on Tolkien’s notes on the Feanorian Alphabet from the 1930s, it seems that the reduction at the end of words of (-mb >) -mm > -m occurred earlier, and medially it happened fairly late in the languages’s phonetic development (PE22/35-36):

Final [mb] became [m] in earliest Gondolic, probably before Y.S. 300. Hence t and y interchange finally, y transcribed m predominating: thus Gond. lham “tongue”, Nargothrondic and Feanorian dialects lhamb until later ...

In later Gondolic (after Y.S. 300) medial mb > mm. The letter t thus became used = [mm] of any origin (PQ pm, bm, sm, mm, mb) and is usually transcribed mm. Thus w]F7 [bār] “home” transcribed bár, pl. `B=tl`B7 [imméı̯r later iméı̯r] transcribed i·mmeir. Before or after consonants in compounds &c., [mm] > [m], usually recognized in spelling: so archaic amb “up” + rhūn “rising” > [ambrūn > amrūn]: written ]y7.F6, transcribed amrún.

In post-Exilic, Toleressean, [mm] generally > [m] but this was not recognized usually in spelling.

It is unclear if this last statement (“not recognized usually in spelling”) applies to Sindarin. It seems that mm is slightly more common than m in Noldorin words than it is in Sindarin, but there is enough variation that it is hard to say. Word-final -mm with no -m variant occurs only very rarely, however. The only example I can find is N. glamm (Ety/GLAM), which appears in Sindarin as glam (WJ/391, 416).

Conceptual Developments: Examples like G. cam(m)a- “stoop” (GL/25) and ᴱN. am(m)arth (PE13/137) demonstrate this sound change occurred even in the earliest conceptual stages in the language, though the phonetic development in Gnomish is complicated by the fact that medial mb was preserved: G. cam(m)a- has a variant form camba-.

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