They are usually given the kind of stereotypical "squeaky, broken english" voice that is sort of used for any of the little "evil races" goblins, kobolds, etc. I mean it varies from creator of course, but from what I've seen, think like Gollum's voice, or the skaven example, with varying degrees of levity worked into the voice. But no there isn't any "official" voice type that I've ever seen, just sort of an unspoken agreement on a general style of voice for them.
I'm usually hearing them in audiobooks, which don't go too heavily into audio manipulation, so it usually just boils down to "high pitched voice, kind of like Gollum, but also a bit different"