Dame:
Check this link about
A Beginner's Guide to Final Cut Pro's Audio Filters.
You'll find the topic
Issue #1 - "Excessive Hiss - Use 3 Band EQ's". Use it as reference, use that filter on your audio and increase lower frequencies till you feel happy with the result.
As Michael said that's non destructive. Your original audio is allways safe and you can delete/change the filsters as you want.
Before mastering your movie, you can export a part of the audio as AIFF and create an audio CD to test it on other equipment/CD player to check if it's good (I don't know what monitor audio system you have in your edition station).
And learn from the examples, too much caffeine is no good for people. ;D
Hope that helps!
Alberto