How to quickly remove unnecessary sounds in FCPX?
There are 2 methods that might help with this.


Press the I and O keys to mark a range around the audio you want to fix. Then press CTRL+OPT+L and type a number value for the amount you want to the audio to drop (eg:-60db)
Make your keyframes, select them, then OPT+SHIFt+C to copy them, then OPT+SHIFT+V to paste them where you need them.


Method 1 is probably more flexible as you can customise the range if need be.