We'd REALLY love to do more of the voiceovers! :(
And we're constantly being bombarded about VO in other languages, too, especially much so from our own country. Sadly, like people already pointed out, VOs are extremely expensive, and the amount of text in the game is staggering, so the budget simply can't handle it at this time :\ Trust me, it's torturous for our sound guys to choose what to voiceover, and what will have to remain silent. They'd really love for the world to always talk.
Don't lose hope though, more voiceovers are likely to be added in future as parts of DLCs or enhanced editions.
Also, some of the voiceovers may actually be missing currently because of the bugs. I think that's what happened to the certain companion's dialogue you're talking about: all dialogues are supposed to be either fully voiced or lack VO completely. There isn't supposed to be any partially-voiced dialogue except for a single occasion with Nocticula somewhere closer to...
Read more