OS. initial syllabic [m], [n], [ŋ] became [am], [an], [aŋ]; [{ṃṇŋ̣}-] > [a{mnŋ}-]

OS. initial syllabic [m], [n], [ŋ] became [am], [an], [aŋ]; [{ṃṇŋ̣}-] > [a{mnŋ}-]

In Sindarin and Noldorin, syllabic nasals at the beginning of words developed the vowel a, so that [ṃ-/ṇ-/ŋ̣-] became [am-/an-/aŋ-]. A clear description of this sound change occurs in the notes on ambar vs. umbar from the 1950s:

... in some case, usually words of strong or emotional meaning or important names, the nasal became syllabic, and then in Quenya > um [from ] (vocalic usually > an; vocalic ŋ̣ > except before gw where it > ) ... In Sindarin a similar development (in similar cases) took place, but syllabic m, n, ñ all had the same vowel am, an, ang (PE17/124).

There a number of examples this phonetic development in both Sindarin and Noldorin:

Some of the above examples explicitly attribute the sound change to Old Noldorin or “Primitive Sindarin” [Old Sindarin]. David Salo also attributes this change to Old Sindarin in Gateway to Sindarin (GS/§4.40). Similar sound changes occurred in the history of Welsh, where short syllablic [ṃ] and [ṇ] from Primitive Indo-European evolved to [am], [an] by the Britannic period (WGCH/§62i). The Welsh changes were more pervasive, however, since syllabic nasals could appear medially in Primitive Indo-European words, not just initially.

Conceptual Development: In Gnomish and Early Noldorin, the development of syllabic nasals was different, in part because like Primitive Indo-European, Tolkien’s conception of Primitive Elvish in the 1910s and 1920s allowed them to appear both initially and medially in words, functioning as ordinary vowels. As described by Roman Rausch in Historical Phonology of Goldogrin (§3), short [ṇ] became [in] and long [ṇ̄] became [an]. For example:

This can also be seen in the vowel gradations in some Gnomish present vs. past tenses, because in the primitive past tense the syllabic nasal was lengthened:

As pointed out by Roman Rausch, apparently short [ŋ̣kʷ] > [uŋkʷ]:

It is unclear whether [ŋ̣] > [uŋ] generally or only before labialized velars, but if I had to guess it would be specifically before labialized velars, with [ŋ̣] > [iŋ] more generally (as was the case with Quenya, see above). It is also possible that (like Quenya) [ṃ] > [um], but there are no examples. Apparently, however, this did not happen if the syllabic nasal was long:

Based on these examples, it seems that syllabic nasals in Gnomish behaved very similarly to the corresponding Early Qenya developments, except that in Gnomish short [ṇ] > [in] whereas in Early Quenya short [ṇ] > [an]. There aren’t enough examples of syllabic nasals to determine the Early Noldorin development of the 1920s, but presumably it was similar to Gnomish.

Reference ✧ PE17/124 ✧ for example: S. amarth < ammarth < ✶ṃbart(ă)

Phonetic Rule Elements

[ṃ-] > [am-] ✧ PE17/124 (*ṃ- > am-)
[ṇ-] > [an-] ✧ PE17/124 ( > an-)
[ŋ̣-] > [aŋ-] ✧ PE17/124 (ŋ̣̃ > ang-)

Phonetic Rule Examples

ŋ̣golodō > aŋgolodo ŋ̣- > aŋ- Ñgolodō > S. Angoloð ✧ PM/360
ṃbar > ambar ṃ- > am- ṃbar > OS. ambar ✧ PE17/104
ṃbart > ambart ṃ- > am- ṃbart(ă) > ammarth > S. amarth ✧ PE17/124
ṃbartā > ambartā ṃ- > am- mbartā > ṃbartā > ambarta > S. amarth ✧ PE17/104
ṃbartā > ambartā ṃ- > am- mbartā > ṃbartā > S. amartha- ✧ PE17/104
ṃbono > ambono ṃ- > am- ṃbono > S. amon ✧ PE17/93
ṃpala > ampala ṃ- > am- KWAL > ᴸON. amphala ✧ PE18/92
ṇdūnē > andūnē ṇ- > an- NDŪ > S. Annûn ✧ PE17/64

ON. initial syllabic [m], [n], [ŋ] became [am], [an], [aŋ]; [{ṃṇŋ̣}-] > [a{mnŋ}-]

See OS. initial syllabic [m], [n], [ŋ] became [am], [an], [aŋ] for discussion.

Phonetic Rule Elements

[ṃ-] > [am-] ✧ PE19/20 (mb- > amb-); PE19/20 (mb- > amb-)
[ṇ-] > [an-] ✧ PE19/20 (nd- > and-)
[ŋ̣-] > [aŋ-] ✧ PE19/20 ([ŋ]g- > ang-); PE19/20 (ŋg- > ang-)

Phonetic Rule Examples

ŋ̣go > aŋgo ŋ̣- > aŋ- ᴹ√ÑGYŌ/ÑGYON > ON. ango ✧ Ety/ÑGYŌ
ŋ̣gole > aŋgole ŋ̣- > aŋ- ᴹ√ÑGOL > N. †angol ✧ Ety/ÑGOL
ŋ̣golonde > aŋgolonde ŋ̣- > aŋ- ᴹ√ÑGÓLOD > N. Angolonn ✧ Ety/ÑGOLOD
ṃbatʰ > ambatʰ ṃ- > am- ᴹ√MBAT(H) > ambath > N. amath ✧ EtyAC/MBAT(H)
ṇdūnē > andūnē ṇ- > an- ᴹ✶ṇdūnē > N. annûn ✧ Ety/NDŪ

G. syllabic nasals became [in] or [an]; [ṇ|ṇ̄] > [in|an]

Phonetic Rule Elements

[ṇ] > [in]
[ṇ̄] > [an]

Phonetic Rule Examples

fṇta- > finta- ṇ > in ᴱ√fṇt > G. fintha- ✧ GL/35
ṇdra > indra ṇ > in ᴱ✶ṇdr > G. in(d)ra ✧ GL/51
jṇta- > jinta- ṇ > in ᴱ√yṇt > G. intha- ✧ GL/37
ṇ̄dai > andai ṇ̄ > an ᴱ✶ṇ̄dai > G. annai ✧ GL/19
ṇ̄t > ant ṇ̄ > an ᴱ✶ṃ̄t- > G. ant ✧ GL/19
fṇ̄t > fant ṇ̄ > an ‽ᴱ√fṇ̄t > G. fant ✧ GL/34
jṇ̄ta > janta ṇ̄ > an ᴱ√yṇt > G. gant ✧ GL/37
jṇ̄tā > jantā ṇ̄ > an ᴱ√yṇt > G. gantha ✧ GL/37
mbṇ̄ð > mbanð ṇ̄ > an ᴱ√mᵇṇřṇ > G. band ✧ PME/58
xṇ̄ða- > xanða- ṇ̄ > an ᴱ√χ̑ṇđ > G. †hanna- ✧ GL/48