I was watching Dirty Jobs and Mike Rowe was deep in a coal mine when he said, "I feel like I'm going to turn the corner and see Cerberus any minute now." One problem, he pronounced it sir-re-bus. One of the coal miners corrected his pronunciation and what followed was a funny little exchange until Mike realized that this coal miner deep in the earth knew more about classical mythology than he, the host of a famous show, did.
But sometimes you do run into names that are hard to pronounce. Take Byggvir, for example (you know, the Norse god of barley? Come on! Who doesn't know that? He's Freyr's servant and without him, how would the Norse gods get drunk?). If you are a teacher, you don't want to mispronounce them in front of the class (chances are, they won't know either, but just in case). For the purists among us, it is a matter of pride. So, where do you go to get proper pronunciations?
If you are looking for the most comprehensive guide for all world mythologies, you need to go here: http://www.pantheon.org/miscellaneous/pronunciations.html.
If that doesn't help, you can try http://www.dictionary.com/. This place will sound out the word for you if you hit the speaker button. Don't count on it to have to more obscure names, like Byggvir.
For that matter, you can also try http://www.godchecker.com/. While they do have Byggvir, they only say COMING SOON for the pronunciation guide. I checked around, they say this quite a lot for the more obscure ones.
Of course, it is hard to blame these sites for not having Byggvir's pronunciation. Spiritus Temporis, which has a page titled "Everything on Byggvir" also doesn't have how to pronounce it. http://www.spiritus-temporis.com/byggvir/
However, I did find a place called Byggvir's Big Beer Cup, http://www.rennfestbeercup.com/who-bygvvir, and they had the verse from the Poetic Edda that talks about him:
"Byggvir is my name,
all gods and men
call me nimble;
and here it is my pride
that Odin's sons drink
ale all together."
But they don't don't have how to pronounce it. I guess "everything" doesn't mean what it used to.