Currently, voice lines are in classified as "Sound effects", so when we lower sound effect volume we also lower the voice line volume.
I know there aren't a lot of voice lines in this game, but for those that are, I for one would like to request the audio options be updated so we can listen to just them. This would let us get a handle on what characters like Mrs. Slitherss and the caverns are saying; sometimes the other sound effects as well as the music get in the way.
Oh, who am I kidding? I ask this so that I can record voice lines and then use them in fan-edits/animations.