When I do voice mods, I almost always try to amplify the liens so that their peaks hit or go a little past the 1.0 mark. Or, if it's mostly quiet but there are a few big peaks, I'll amplify everything but the peaks first and then amplify.
Once you start going above 1.0, the audio corruption sets in, and you lose intelligability, but much lower and the voices are tough to hear, especially with music on
Here's an idea: Try importing a voice clip of the character you're making the mod for. That way, you can compare both clips and decide if the custom one is too loud or quiet and make adjustments accordingly. It helps to have headphones, too.
As for adjusting the volume in general, on the left side of the audio track, there are two sliders: one for gaining (top) and one for panning (bottom). The gain slider is what you'll want for the volume. Pretty simple, really.