Old Vauqun/Sound Changes

Old Čeuň is a direct descendant of Proto Čeuň-Adzovŭd, and due to it's location at the easternmost extreme of PČA languages it earns the namesake of the proto language, along with its westernmost sister Proto Adzovŭd. Note that IPA will be used for all transcriptions of the languages in question, regardless of the phonemicity of their standard writing systems.

Čeuň-Kfimea Dialectal
PČA maintained a distinct Čeuň dialect for some time before the split. Various isoglosses are shared with P Kf (esp. E Kf languages). These dialectal rules include:

Expansion of I-Umlaut

[j] now triggers an I-Umlaut (explained in further detail on the PČA morphophonemics page)), only affects the syllable immediately preceding the [j], unless a [w] intervenes in which case the umlaut does not occur: [a æ o u] > [æ e e ɨ] / _$(C)(V)j !_...[rw]...j

Rounding of ɨ
 * fatjo > fætjo
 * but nawkjo > nawkjo

[ɨ] and [ʉ] alternated widely in PČA dialects; in the Č-Kf dialect ɨ became ʉ when preceded by [w] or when in a closed syllable Palatalization of Velar Spirants
 * gnɨːwo > gnʉːwo
 * xɨrket > xʉrket

[x] [γ] (which might have had the allophones [ç] [ʝ] in palatal environments already) became [ʃ] and [ʒ] before [i iː e eː] and [j]. This became phonemic when various vowel shifts occurred later (after the era of PČA unity) Assibilation of Fricatives
 * suxidvẽː > suʃidvẽː

Dental and velar fricatives become their respective alveolar and postalveolar sibilants in root-initial consonant clusters. Both Čeuň and Kfimea languages feature further reduction of the dental fricatives but other than this the sound changes are largely handled differently by each language group: [θ ð x γ] > [s z ʃ ʒ] / #_C
 * θke > ske
 * ðgaru > zgaru
 * xnɨ > ʃnɨ

Pre-OČ
Palatalization of Stops

Dental and velar stops become affricates before front vowels and [j], and when after [j]: [t d k g] > [ts dz tʃ dʒ] / _[i e iː eː j] OR [j]_. [j] then typically disappears and causes gemination, unless certain lexical pressures keep it in place. [i] (but not [iː]) behaves the same way if it precedes another vowel. The affricates that geminated sporadically underwent metathesis and lost their length. This only happened word initially or after a vowel. Intervocalic Lenition of Nasal Consonants
 * sĩːkje > sĩːtʃːe > sĩːʃte
 * xrevdie > xrevdzːe
 * totẽːk > totsẽːk
 * ajk > atʃː > ! atʃ (irregular)

Nasal consonants lenit to glides or fricatives between vowels: [m n] > [ṽ j̃] / V_V Diphthongization of Nasal Vowels
 * sreme > sreṽe
 * venɨ > vej̃ɨ

Nasal vowels fracture into a vowel and nasal off-glide, the latter of which potentially assimilates into the following consonant (note that in PČA nasal vowels were not followed by nasal consonants or glides, also note that all geminates mentioned simplify when not followed by a vowel)

[ãː ũː ẽː ĩː] > [aw̃ ew̃ ej̃ ej̃]

[j̃ w̃] + [v b] > ṽː

[j̃w̃] + [d g] > [nː ŋː]
 * vẽː > vej̃


 * kɨnũːt > kɨnew̃t

Deletion of Word Final Stops
 * dãːvo > daw̃vo > daṽːo
 * mĩːgæ > mej̃gæ > meŋːæ (contrast mĩːge > mej̃dʒe > men(d)ʒe)

Title says it all :P [+stop] > ∅ / _ # Dephonologization of Dental Fricatives
 * totsej̃k > totsej̃
 * xʉrket > xʉrtʃet > xʉrtʃe

Dephonologization of <ɨ> <æ> <ʉ>

The first shift in the oral vowel system was the loss of <æ> and <ɨ> as phonemes distinct from and. <æ> became [a] when in a closed syllable or when short and preceded by a velar consonant (æ > a/ _C$ and [+velar] _ !_ː); elsewhere it became [e]. <ɨ> became [i] when long or when after [j]. Elsewhere it became [e]. <ʉ> became [u] in open syllables or when short and preceded by a velar consonant (ʉ > u / _C$ and [+velar] _ !_ː), [o] elsewhere.

Note that new instances of [e] and [i] did not trigger palatalization in the consonants they followed.

U-Umlaut
 * æffu > affu
 * γæte > γate
 * kæːwo > keːwo
 * eːkjæ > eːtʃːæ > eːʃtæ > eːʃte
 * gnʉːwu > gnuːwo
 * fɨro > fero
 * rʉffʉs > roffos
 * xʉw > xuw > xuː

[w] [w̃] and [u(ː)] now trigger rounding of preceding vowels similar to the historical fronting effect of [j] and [i(ː)]: [w] and [w̃] cause the vowel in the syllable immediately previous to round, and [u(ː)] triggers the rounding of all vowels in a word when it appears word finally. An intervening [j] or [j̃] protects preceding vowels from a u-umlaut. ([a e i] > [ɔ ø y] /_(C)$...[w w̃]
 * zaru > zɔru

Word Final Vowel Shifts

(none of these shifts apply in monosyllabic words)

Short high vowels disappear word finally, and short mid-high vowels rise to take their place

[i u] > ∅ / _ #, [e o] > [i u] / _ #

Note that this causes a significant shift to the already rather impacted gender marking system for adjectives and pronouns. It also creates a series of syllabic consonants, and introduces syllable-final nasal stops
 * togru > togr̩
 * tɨbγe > tebʒe > tebʒi
 * tɨbγæ > tebγe > tebγi
 * eːkje > eːtʃːe > eːʃte > eːʃti
 * eːkjæ > eːtʃːæ > eːʃtæ > eːʃte > eːʃti (later > eːkki by analogy with words like tebʒi / tebγi)
 * keːro > tʃeːro > tʃeːru
 * ɶffu > ɔffu > ɔf
 * æffo > affo > affu
 * ræki > retʃi > retʃ
 * ʃnʉnu > ʃnunu > ʃnun