As I understand it, there is no official pronunciation for many of the words/names that are made up for the story, but it helps to know/figure out how to pronunce these things anyway.
In any case, am I the only person who applies a french pronunciation to Moirail? Yup, I pronounce it /mwɑ'ʁɛl/.
Even before I discovered that Leijon was taken from old Swedish I pronounced it /leɪ'joʊn/, which is probably pretty close to how it's actually pronounced.
Anyway, I think that the actual pronounciations of things are probably based on what words they're taken from, like Leijon is Swedish, so naturally it would have a Swedish pronunciation.