In (Old) Sindarin both long [ā] and diphthong [au] developed into [ǭ], and then later this [ǭ] became [au]. The clearest description of this change appeared in notes from the late 1960s:

P.E. *au and *ā had both become õ (open as in English pronunciation of au), and then > au, which however only remained in stressed monosyllables, and otherwise again reverted to o (NM/367).

Here the õ (IPA [ɔ]?) almost certainly represents the open o Tolkien usually represented as [ǭ] (see below). For the more limited case of [ā] > [ǭ], Tolkien mentioned this sound quite frequently. For example, it is discussed in the second version of Tengwesta Qenderinwa (TQ2) from around 1950, written during the transitional period between Noldorin to Sindarin:

In the earliest Noldorin ē, ę̄, ā, ǭ, ō appear as ī, ę̄, ǭ, ǭ, ū; later ę̄, ǭ coalesce with ai, au (PE18/96).

There are similar descriptions of this phonetic development elsewhere in Tolkien’s later notes on Sindarin:

• In Sindarin ... these newer long vowels followed the same development as the original long vowels ... ā > ǭ, au ... (PE19/91).
• ... stressed ā, au both produced S au (PE19/107).
• NB borrowing was after Sindarin change of ā > ǭ, aw (PE17/45).
• Welsh is usually[?] ām (e.g.) > aum̌ > au(v) > au, but Sindarin ām > ǭm, ǭm̌ > aw(v) (PE17/99).

Examples of this Sindarin phonetic development are easy to find, especially if you also consider that later [au] generally became [o] in polysyllables:

• ✶yānā [> *yǭna] > S. iaun “wide, extensive” (PE17/42).
• ✶lāmina > lǭ́m̃en > S. loven “echoing”, vs. North Sindarin lomin (PE17/133).
• ✶wā- > gwǭ- > S. go- “together” (WJ/364).

There are a few aberrant developments where ā seems instead to produce oe, but these all originate from the combination āy:

• ✶gāyā > S. goe “terror, great fear” (PM/363).
• ✶wāyā > gwoe > S. gwae “wind” (PE17/34).

These can be explained by the diphthongal development whereby [ǭi] became [oi], and then later [oi] became [oe].

Clear examples of ancient [au] > [ǭ] > [au] are hard to find in Sindarin since the end result was often no change from the primitive form, but some examples exist in Noldorin which had very similar developments (see below).

Conceptual Development: As discussed by Roman Rausch in his Historical Phonology of Goldogrin, the Gnomish development of long ā is much simpler: it just became long ō (HPG/§1.1). In Tolkien’s very early chart of the development of Gnome Vowels, at first long [ā] was preserved: â > â (PE15/13). But in the Gnomish Grammar Tolkien said:

In most cases of monosyllables the development ā, ē, ō, ī, ū > ō, ī, ū, ai, au ... (GG/14).

The examples of the phonetic development [ā] > [ō] are numerous in the Gnomish Lexicon, especially after including the cases where the long [ō] had shortened:

• ᴱ✶ŋguār > gwōr- > G. gôr “each, all” (GL/41).
• ᴱ✶mᵇāsḷ > G. bothli “oven” (GL/23).
• ᴱ✶eđusmānī > G. Edhofon (GL/20).
• ᴱ✶qāmē > G. côma “disease” (GL/26).

As pointed out by Roman Rausch in his Historical Phonologies of Ilkorin, Telerin and Noldorin around 1923, by the 1920s it seems that long ā > au in Early Noldorin instead (HPITN/§4.2.3). For example:

• ᴱN. pau “sickness” vs. ᴱQ. qáme (PE13/152); compare to G. côma above, and earlier ᴱN. côm (PE13/123).
• ᴱN. maur “good” vs. ᴱQ. mára (PE13/150); earlier forms G. mora (GG/10, GL/56) and ᴱN. môr (PE13/122).

Based on these last two examples, it seems Tolkien still used the older Gnomish phonetic developments in the Early Noldorin Grammar from approximately 1920 (PE13/119-132), but switched to the newer development in the Noldorin Word Lists circa 1923 (PE13/133-156): the word lists have the deleted form cóm “disease” (PE13/140) vs. replacement pau “sickness” (PE13/152), and deleted mór vs. replacement maur “good” (PE13/150). However, in the Gnomish Lexicon Slips (which were probably composed between 1918 and 1920) Tolkien already had mawr “good” (PE13/115) and bawr “house” < mbāră (PE13/116), so Tolkien seems to have been considering this revised phonetic development even earlier.

Noldorin Developments: The new development [ā] > (ultimately) [au] was well established by the Noldorin of the 1930s, and Tolkien elaborated on it in the first version of Tengwesta Qenderinwa (TQ1) from the mid-1930s:

In the earliest recorded Noldorin, [ē, ǣ, ā, ǭ, ō] remained distinct though changed to [ī, ei, ǭ, ou, ū]; later ei, ou products of ǣ, ǭ became ai, au coalescing with original ai, au (PE18/46).

These sound changes also appear in the Comparative Tables along with entries indicating primitive [au] > [ǭ] as well (PE19/25), much like the note on Old Sindarin mentioned above (NM/367). In Noldorin, both these sounds later passed through [ǫu] before becoming [au] (PE19/25). Compare this to the above note from the second version of Tengwesta Qenderinwa, which is more representative of Sindarin development:

In the earliest Noldorin [read: Sindarin] ē, ę̄, ā, ǭ, ō appear as ī, ę̄, ǭ, ǭ, ū; later ę̄, ǭ coalesce with ai, au (PE18/96).

The Noldorin intermediate stage [ǫu] is at least partly preserved in Sindarin. While Sindarin had original primitive diphthong [ou] > [ū], any later diphthong [ou] produced by other later sound changes would become [au] (PE17/99), which is exactly how such later diphthongs developed in Noldorin. In the Noldorin of The Etymologies, however, it seems that original primitive [ou] was also preserved, which along with [ou] of later origins ultimately became [au]. Thus, while Sindarin and Noldorin generally produced the same results (with the exception of primitive diphthong [ou]), the developments went through different paths. For the forms that ultimately produced [au]:

1930s-40s	PQ	ON		N
Noldorin:	[ā] >	[ǭ] >	[ǫu] >	[au]
	[ǭ] >	[ǭ] >	[ǫu] >	[au]
	[au] >	[ǭ] >	[ǫu] >	[au]
	(late diphthong)		[ou] >	[au]
	[ou] >	[ou] >	[ou] >	[au]
1950s-60s	PQ	OS		S
Sindarin:	[ā] >	[ǭ] >		[au]
	[ǭ] >	[ǭ] >		[au]
	[au] >	[ǭ] >		[au]
	(late diphthong)		[ou] >	[au]
	[ou] >	[ū] >		[ū]

[ǭ] vs [ō]: Tolkien only rarely wrote ǭ in The Etymologies, and did so only in primitive forms, such as: ᴹ✶rǭda > N. rhaudh (Ety/ROD; EtyAC/ROD); ᴹ✶kǭro > N. Caur (Ety/KOR; EtyAC/KOR); ᴹ✶mǭri > N. maur (Ety/MOR, EtyAC/MOR). All these examples are from the Etymologies, Addenda and Corrigenda (EtyAC; VT45, VT46); Christopher Tolkien used only ō in The Etymologies as originally published in the Lost Road (LR/341-400). For any intermediate ON developments, J.R.R. Tolkien himself wrote only ō instead of ǭ, both in The Etymologies and elsewhere. For example:

• ᴹ√SAB [> *sāba] > ON. sōba > N. saw vs. ᴹQ. sáva “juice” (Ety/SAB).
• ᴹ✶tārā > ON. tōra > N. taur vs. ᴹQ. tára “lofty, high” (Ety/TĀ).

It is my belief that these ON long ō are simply shorthand for [ǭ] (see below).

Tengwar Evidence: Other clues to the Noldorin phonetic developments can be found in Tolkien’s discussion of the use of the Feanorian Alphabet for Old Noldorin from the late 1930s. In the first draft of these tengwar documents Tolkien had:

The Noldorin change of Eldarin [ā] > [ō] (PE22/15).

While it is possible Tolkien briefly reverted to the Gnomish development, it is more probable that he was simply using [ō] as a short-hand for [ǭ] (< [ā]) as I indicated above. There would be no conflict with original primitive [ō], since such primitive [ō] > [ū] very early in Noldorin. Later versions of tengwar notes are consistent with the Noldorin phonetic developments of The Etymologies, with the most relevant portions underlined:

], l [a], [e] were always short, since PQ ā, ē has become ō, ī: hence their names a-stinta, e-stinta “short-a, short-e”. In transcription the ON method is employed, where the exact ON spelling is represented. But in the grammar and Etymologies long vowels are marked with a macron (as ō, ū) (PE22/27).

   é called (amba-wai) “raised-w”. This was frequently used above letters to denote a following w: thus dé = khw, 7é = rw, etc. Its most regular use was to express the diphthongs au̯, and the archaic ou̯ from PQ ǭ which later became au̯: thus ]é [au̯] (and archaic hé [ou̯]). These were seldom written ]., or h. (PE22/27).

[o͡u]: ON au, ō or ŏ + ʒ, χ. archaic h. later > [au] q.v. (PE22/39).

The Noldorin origin of this is seen in use of h = o. This is derived from the Kornoldorin use. Thus ʒā name of h became with loss of [ʒ] ā > ō with N. change of ā > ǭ. Hence h called [ō] but having no function was adopted as sign = [o] (PE22/48).

This last quote further supports the idea that Tolkien’s use of ō in ON was a shorthand for [ǭ], though it may instead indicate a variant development or another intermediate stage: [ā] > [ǭ] > [ō] > [ou] > [au] (unlikely but not impossible). For further discussion of the conceptual history of the phonetic developments of [ou], see the entry on how [ei], [ou] became [ī], [ū] (in Sindarin but not necessarily Noldorin).

Other than the variant developments for ancient [ou], it seems the Noldorin changes were very similar to later Sindarin.

GS/§4.58 GS/§4.98 @@@ probably ou > ǫu @@@ Tolkien may have abandoned au > [ǭ] since it was not mentioned on PE22/27 and there is no evidence of it in later writing

HPG/§1.1