This is typically, and somewhat lazily, achieved using a compressor/limiter - which will have settings for:
threshold (the level at which the signal will start to be squashed)
ratio (the amount the signal over the threshold will get squashed)
attack (how quick it responds to the signal going over the threshold)
release (how quickly to stop squashing once the signal is back under the threshold)
make up gain (how much to raise the volume to counteract the compression you did)
100:1 at -10dB with fast attack and release times would be a brickwall limiter (it is really squashing everything above -10dB)
3 or 4:1 at around -24dB with quite fast attack and slower release times with a few db of make up gain might be a more gentle compression
assuming you are hitting broadcast specs and material is close to correct
i think the fcp compressor's make up gain is 'preserve volume' from memory
you can make audio sound pretty bad pretty quickly with compression though....
Audio post pros use compressor limiters as a matter of course. As a mixer I don't think FCP7 is a useful tool for broadcast sound post and the compressor limiter plugins are not great. They are also clip based not track or bus based so again not much use.
A really good limiter can gain reduce peaks by up to 6db without sounding like they are actually doing anything. But sound post is not just about whacking a limiter on.