The voices are actually alright. It's the English translation that doesn't flow well at all.
Here's an example:
https://www.youtube.com/watch?v=pcoRb7B7PCM
Japanese (very rough and very literal translation):
English:
The biggest problem I have with this is Ignis' line. He says "Indeed" but what is he agreeing with exactly? It's not what you would say to someone who is asking you something: "I ask that you do this for me Ignis." In this case, "Leave it to me" flows a lot better than "Indeed".
But there are some other nitpicks. Note that the Japanese script, even with my shoddy translation, shows more emotion than the English script. Japanese Prompto is still clearly not comfortable with speaking to the king, while English Prompto is just okay with it. Japanese Regis seems very caring of his son and actually seems worried. He speaks to him and his friends like a normal (Japanese) father would, while English Regis delivers a grand and majestic speech in language fit for a king but not for a father.
Of course, this is all nitpicking, but I found many weird translation choices like these back in Episode Duscae and it looks like this hasn't improved. The 50-minute gameplay video we saw was riddled with small things like these that just don't make the English dub enjoyable to me at all. Sadly, even with the Japanese VA we'll still get the English translation in the subtitles,
inconsistencies and all. I can more or less follow the flow of the Japanese lines, but I feel sorry for those who can't and are stuck with this translation.