FCP’s audio filters cannot repair major audio problems. Read that again if you didn’t get it. If you have excess background sound, close the doors and windows, reposition your mics, put up sound blankets, switch shoot locations or do anything else possible because no audio process or filter is really going to do a great job at “fixing” that issue. If you encounter hum while setting up for the next shot, track it down and fix it before shooting. You used an on-camera mic and the dialogue is low level and full of echo? Sorry, you should have shot it correctly. There isn’t an audio filter or box in the world that will turn that audio into something high quality.