There isn't any way to change the volume of specific people, but you can tell them to turn their mic up. A lot of the time peoples' mics are not set up to where everyone can hear them. It takes some playing around with to hit that sweet spot. They can use a mic test software online or a local program to test their mic volume.
I do not talk a lot during game play, normally because I have family in the house that doesn't want to hear me yelling and just in general I do not like talking over people. However, you can turn down your master volume and then turn your actual volume up. This will help with this issue, but the people you could hear clearly before will be loud as hell.