
A recent thread on the BOLS Lounge made me think of this topic. Different communities seem to have their own way of pronouncing words in the 40k universe. Here's some common ones:
Lasgun- According to Graham McNeill, it's LAS-gun with the A as in apple. I have heard people insist that it is LAYS-gun, as in laser. I actually used to think that they were right because of the laser pronunciation, but I always pronounced it LAS because LAYS just sounds silly.
Tzeentch- I haven't heard this one mispronounced very frequently but apparently some people start the word with an X sound. It's simple to pronounce, really. Say the word "pizza" and take the sound from the double z (in Italian double z is pronounced like a "tz" and some times like a "dz"). I believe Chinese and Japanese both have similar sounds.
Slaanesh- I always pronounce it SLAAN-esh with the A as in apple. However, I've heard a couple of times people saying Slaan-EESH which doesn't seem right to me. I figure that the doubled vowel indicates stress.
Tyranids- I've heard people pronounce the Tyr as in Tyrant so they become TIE-ra-nids. I say TEER-a-nids and I think the majority agrees with me on that one.
Nobs- I've always pronounced it as in knob, but some new guys in my group say "nohb" as in noble. That makes sense actually as an Ork nob is simply short for Noble, but Lexicanum disagrees and says that it is knob.
C'Tan- K-tan. It's a hard "cuh" sound. I've heard of people saying see-tan but Graham McNeill disagrees. Think of Cthulhu. But not for too long lest ye go mad.
Any that I missed?
Edit: Be sure to check out the comments, there's some good information there.