It sounds like the compressor/limiter filter would work out for you as something you should play around with. Read the description in the Final Cut Manual, as I just did. I’m actually using it right now cause i have a finished short film I’m adding to a reel and the levels were mixed with some loud car crashes at -1db and soft dialogue -12 to -18. Used the com/limit to bring down the crashes to -6db and sounds much better now. Hope this helps.