Where does “ṡón” come from?

ṡón (Old Irish) comes from Old Irish són, from Latin sonus, from Latin sonō, from Proto-Italic *swonaō.

Ancestry of “ṡón”, step by step

Step	Language	Word	Meaning
1	Old Irish	són	clitic form of sodain
2	Latin	sonus	sound, noise; pitch; speech; tone, character,...
3	Latin	sonō	—
4	Proto-Italic	*swonaō	—

ṡón — Etymology, Origin & Word History | Etymology Explorer