I recently bought the voice pack. After some technical difficulties, it's now working fine . . . except that there are some moments where the main characters don't speak. For example, at the very beginning when Mel wakes Stella up, Stella speaks, but Mel doesn't. Then when Edward scolds Mel and Stella for being late, he's not speaking. Also, in Pemberly keep, when you can have your characters chat with each other, Galahad only speaks when he's the party leader and he talks with Te'ijal. When any of the other characters talk to Galahad, he doesn't speak. There are other incidents where the characters don't speak as well.

Does anyone else have these problems, or is it just my computer?

or it might be the voice pack itself. maybe the dialogs that doesn't have vice are intended that way.

It's the voice pack. It was recorded using the original script (written before even LoT was finished), and a lot of things in the dialogue changed after recording was complete. Some of the party interaction was not in the script at all, so was not recorded.

Once you actually get into the game, things like this will not happen as often.

Ah, I see. Glad it's not my computer acting up again.

Would there perhaps be a chance of a voice pack upgrade with the missing dialogue put in?

It's very unlikely. Some of the actors were no longer available when we were putting the voices into the game, so their lines couldn't be re-recorded.