I started podcasting with cheap Radio Shack microphones, a couple of mixers, and a lot to learn. Getting everyone to sound like they were in the same conversation took some work.
Levelator became one of the tools I kept using. Drop in a WAV or AIFF file and it evens out the differences between voices. My equipment has improved since then, but I still run the recording through it. Old habits, useful tool.
The Conversations Network created Levelator. I didn't build it; I keep these older installers here because it's been part of my own production setup for years. Singular Software also offers a current Mac edition on the App Store.