The general opinion among the Pagan community is that it's pronounced "Sow-en," which would seem accurate, given the etymology of the word. It's from the Gaelic “Samhuin,” which means either the end or beginning of summer, depending on which scholar you ask.
If you'd like to hear how to pronounce a lot of words you'll see on this site and other Pagan web pages, add this to your list of Really Fun Linkage: The Pagan Talking Dictionary. It features sound files of proper pronunciations of words commonly found in the Pagan and Wiccan lexicon. This is a great resource, so be sure to check it out!
Follow Pagan/Wiccan on Twitter or Join Me On Facebook!