Are the voiceovers at a low volume initially, before you introduce them into the iMovie program? Or does it change once you have combined them?
I am not a Mac user, so any help I can offer would be with the few programs that are cross platform with Windows. Audacity is a free audio editing program that can be used in both systems, and it has the capability of increasing gain (volume) on audio. http://audacity.sourceforge.net/download/mac
If you are working with AC3 format audio, you'll need the ffmeg import/export library as well, which I hope works in OSX