I'd rather they just keep the games "silent." Too many possible issues with voicing. I think the biggest thing is the fact that it is a handheld, as has been mentioned previously. A lot of people won't be in a position to be playing with the volume up. What's the point of dialogue that can't be heard?
I did, however, take interest in the idea of adding background sounds/noises or maybe emotions to trainers. That could work, but would need some visual assistance. For example, after you were to defeat a trainer, have them make some type of displeased grunt or sigh and have them show some kind of facial expression that shows disappointment. That's what they should focus on with the new games. With the 3D world, it's better they focus on the visual improvements than the audio improvements.