Previously, we used vdiph/ui_4 for [[ui]]; I think the main reason for
that was that I didn’t like how the most common ⟨ui⟩, vdiph/ui, seemed
to almost vanish in “Cuiviénen”. However, vdiph/ui_4 has the curious
property that in some positions, e.g. ⟨uia⟩ in “tuia” or ⟨uil⟩
“tuilindo”, it sounds (to me) more like /ul/ than /ui/. (This also
affects Finnish, which seems to be the only other language that uses
vdiph/ui_4 [a few other languages also use it for [[ui]] but don’t seem
to emit that phoneme in their rules files] – listen to eSpeak NG
pronounce Finnish ”luiun”, for instance.) I eventually found out that
this can be worked around by substantially lengthening the phoneme
(length 500 seems to work in all positions), but this extreme length
(the absolute maximum is just 511) becomes rather noticeable whenever
the ui is used, including in positions where it had sounded just fine
before. Meanwhile, the more standard vdiph/ui can be made to sound
reasonably well in “Cuiviénen” with a much smaller increment to its
length: 290 (as also in ph_lithuanian) instead of 240 (as in ph_base2)
is enough. In this version, [[uI]] sounds acceptable enough for Elvish
⟨ui⟩ in all positions, as far as I can tell.

This is “a voiceless w, as in English white (in northern pronunciation)”
according to Appendix E of The Lord of the Rings, and so we copy the
[[w#]] phoneme from the English phonemes. I can’t actually hear much of
a difference from the previous [[hw]] (I know what the difference
between [[w]] and [[w#]] should be, but [[hw]] already sounds like
[[w#]] to me), but at least this improves the --ipa output, changing it
from [hw] to [ʍ].

I’m not aware of any statement by Tolkien to this effect, but it just
seems natural to me, and the [ŋ] is at least found in the Omikhleia
Sindarin dictionary [1] (e.g. entry tinc, [tˈiŋk]).

Based on a similar phoneme rule in ph_english.

[1]: https://www.jrrvf.com/hisweloke/sindar/index.html

For ⟨ae⟩, we used to use the Latin [[aI]] phoneme, which sounds like
/ae/. Call that phoneme [[aE]] for us and introduce a separate [[aI]]
one, which sounds more like /ai/ (since Sindarin has both, and they’re
supposed to sound different, though Appendix E of The Lord of the Rings
notes that there is nothing closely corresponding to ⟨ae⟩ in English and
that it may be pronounced like ⟨ai⟩). Furthermore, for ⟨oe⟩, just remove
the TODO – the Latin phoneme is called [[OI]] but sounds more like /oe/
than /oi/, so it’s actually just fine for our purposes. Finally, the
⟨ui⟩ diphthong is copied from Finnish, just like in Quenya.