Personally, I've always kinda translated it in my head as either 'Mix-ee-p't-ulk' or 'Mix-tee-plikt' - generally the latter. I know that it CAN'T actually be 'Mix-tee-plikt', but that's how I see it - and considering that the current version is actually a misspelled version of the name Siegel created (he spelled it 'Mxyztplk'; the current version is 'Mxyzptlk'), I think it doesn't really matter HOW it's pronounced.