There are 4 main steps you can take to salvage nearly any audio.....depending on the end use.
1. apply a de-noise. The Final cut de-noise is OK, but some third party ones like sound soap or izotope are better.
2. apply a de-reverb (if it is from an indoor shoot and you are getting too much room sound). There is a cheap one from crumple pop that does a decent job.
3. Use the eq to adjust. 240khz frequency is the depth of a voice, 500k is the muffled frequency range, 1k is the nasally range, 2k is the rasp, 5k is the 'esssss' range and 7k+ is breathiness or the airiness (also where the hiss lives). Often when you remove noise you also muffle the voice a bit.
4. Finally, you can increase the level via the volume, or by using a limiter or compressor to get the volume back up to a reasonable level. The loudness in FCPX is good as a start.
I've been an audio editor for approx 20 years and this is my workflow.