What is the fastest way to "even out" audio levels in FCP, ie, to make the entire audio "clip" even at -12 dB, or is this even possible without going through the audio clip and manually adjusting the levels? Thanks for the help, this could save me a lot of time.
All you are doing is setting the maximum peak to a fixed setting. That isn't going to make for a mix or balance. Peak levels do not equate to apparent loudness.
Just like grading is a shot by shot interpretation using scopes, audio is also an interpretive art using meters to help and also to check for tech acceptance.
EQ & Dynamics control via compressor/ limiters are how to get both a balance and be within tech acceptance. It is just as much an art as grading or editing so either pass it on to a pro or spend the time to make it right. After all sound is much more than 50% of your final work so either take it seriously or get out of the way and let a sound post pro take over.
Ironically it is the cheapest way to add value to your production and the skill that many editors seem to lack. Ignorance about sound is a constant amazement to me.