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:

I-Umlaut

[j] (in all positions) and [i] (when long or word final) trigger umlaut of preceding vowels, an intervening  blocks the effect from occurring [a o u ɨ] > [æ ø ʉ i]

Rounding of 
 * fɨrokje > firøkje
 * deðgajeːk > deðgæjeːk
 * suːrzi > sʉːrzi
 * vawojeːk > vawøjeːk (not *væwøjeːk)

[ɨ] and [ʉ] as manifestations of  alternated widely in PČA dialects; in the Č-Kf dialect  was ʉ when preceded by [w] or when in a closed syllable (and when the product of an umlauted [u], as discussed above), otherwise it was realized as [ɨ] Palatalization of Velar Spirants
 *  = [gnʉːwo]
 * = [xʉrket]
 * = [venɨ]

[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 (whether it is before or after), unless certain lexical pressures keep it in place. The affricates that geminated underwent metathesis and lost their length (though this did occasionally fail to occur. Intervocalic Lenition of Nasal Consonants
 * sĩːkje > sĩːtʃːe > sĩːʃte
 * xrevdie > xrevdzːe
 * totẽːk > totsẽːk
 * ajk > atʃː > ! atʃ (irregular, "correct" form [aʃt] unattested)

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 never followed by nasal consonants or glides, also note that all geminates mentioned simplify when not followed by a vowel)

[ũː ẽː ĩː] > [øw̃ aj̃ ej̃]

[j̃ w̃] + [v b] > ṽː

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


 * kɨnũːt > kɨnew̃t

Deletion of Word Final Stops
 * dãːvo > daw̃vo > daṽːo
 * mĩːga > mej̃ga > meŋːa (contrast mĩːgeː > mej̃dʒeː > men(d)ʒeː)

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

The dental fricatives

Early OČ
Dephonologization of 

U-Umlaut

[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] or /_...u(ː)#!_...[j j̃] Word Final Vowel Shifts
 * zaru > zɔru

(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