Where does “sõo” come from?

sõo (Old Portuguese) comes from Latin sonus, from Latin sonō, from Proto-Italic *swonaō.

Ancestry of “sõo”, step by step

Step	Language	Word	Meaning
1	Latin	sonus	sound, noise; pitch; speech; tone, character,...
2	Latin	sonō	—
3	Proto-Italic	*swonaō	—

sõo — Etymology, Origin & Word History | Etymology Explorer