NOTE: Edited to reflect timing issues, on whether this change took place before or after i-affection, and the implications on plural forms.

S. [x], [ɸ] vocalized between a vowel and [s], [θ]; [Vx{θs}|Vɸ{θs}] > [Vi{θs}|Vu{θs}]

NOTE: Edited to reflect different Sindarin vocalization of [ŋ] (vanishing with compensatory lengthening).

S. [ɣ] vocalized before [l], [r], [m], [n]; [V{ɣ}{lrmn}] > [Vi{lrmn}]

NOTE: Edited to reflect possible Sindarin development of [ŋ-] > [ŋg-] > [g-]

OS. initial and intervocalic [ŋ] vanished; [ŋ-|VŋV] > [ø-|VøV]

S. short [a], [o], [u] became [e], [œ], [y] preceding [i]; [{ăŏŭ}{C|CC}i] > [{eœy}{C|CC}i]

S. short [e], [o] became [i], [u] in syllable before final [i]; [-{ĕŏ}{C|CC}i] > [-{iu}{C|CC}i]

S. short [u] often became [o]; [ŭ|uC{uw}|u{mnŋ}] > [o|uC{uw}|u{mnŋ}]

S. medial [j] became [i]; [-j-] > [-i-]

In Sindarin and Noldorin any surviving medial -y- [j] became i at some stage. This is a factor in Sindarin orthography, where i always represents the vowel [i] medially, but at the beginning of a word before another vowel represents the “y”-sound [j], because it is the only place where this sound can appear:

S. [ɸ], [β] became [f], [v]; [ɸ|β] > [f|v]

S. [iu] and [ju] became [ȳ]; [{ij}u|jui] > [ȳ|jui]

Do you think péti "lips" could be a model how CV monosyllables form their plurals? I mean like pé - péti, ré - *réti, thé - *théti, possibly pí - *píti etc

It's a wild stab, I know, it just occurred to me.