> Could be onomatopoeic in origin.

That's right, it clearly has some influence otherwise all these expletives would not be as related as they are.
In Germany cliche exclamation of pain is 'Au-aa!', which is even turned into a telly-tubby-like substantive with little children, like so:
'Hast du dir ein Ow-aa gemacht?'

BTW, I've noticed that 'Youch' is also used in English, usually to express something that doesn't physically hurt, e.g.:
'Youch, that phone bill is gonna hurt!'