The way MY coven pronounces it is "Sow-ain."
Frigging Celts, most tongue twistingly unpronouncable language there ever was. I have a friend who says that's the reason they could never become a force to be reckoned with, their language was too complicated.
I've heard "Sowain", "Sowun", and "Sowune". The only thing everyone seems to be sure of is that "mh" is a single phone like "th" in english.