Without getting too much into the details as they're complex and vast sometimes (eventually) you'll receive audio that is out of phase, likely 180°, if the audio sounds like it was recorded with the mic in a plastic bucket. Another clue is that it'll sound okay when played back from whatever it was recorded on but be very low volume when played back in Final Cut Pro.
There are myriad ways this can happen but the most common is that the audio was recorded on two separate mic's that were improperly spaced apart so the audio they're recording reaches one mic later than the other and causes a comb filter effect.
One way I've experienced this is when on set someone has the brilliant idea of setting up a "back-up" mic close to but not along side the main mic's and rather than helpfully recording it to a separate channel they mix it in with the the main audio feed. I cringe when someone tells me they have "reference" audio as well on a multi-camera shoot because there's the risk the audio guy (or girl) mixed it all down.
One particularly bad case was when someone mixed in an off-camera (and off-set) "back-up reference" shotgun mic on a c-stand along with a TV host's lav. I know!
The out-of-phase audio you can monkey with is when there are two mic's recording the same audio source but those mic's are placed at unequal distances from that audio source and they're recorded to separate channels. Audio bouncing off a nearby wall or something into one mic can cause a similar problem.
The basic result is that one audio channel is canceling out the other (sorta, it's really just reducing the amplitude). The peaks of one meet the troughs of the other's waveform thereby reducing the clarity and volume of the audio. This page has some good visuals explaining it. You can get an idea of the effect by reversing the leads to one speaker on a stereo and playing a VO (Voice Over); it'll sound "funny".
Final Cut Pro doesn't have a way to fix this readily, but one quick check for phased audio (and a fix kinda) is to simply turn off one channel and listen to it to see if it sounds better. If it does: the audio sounds brighter, louder, clearer, it's likely the audio was recorded out of phase.
A quick fix (meaning if the client is there waiting when the audio sounded great when played back on their camera through headphones) is to turn off one of the channels, duplicate the one remaining above or below itself, and then adjust the audio level a bit. It'll be in mono but you can fake that by offsetting the duplicated channel by a few hundredths of a frame (in the Viewer via moving the In point while you hold down SHIFT).
I may post more about this later in greater detail but I thought I'd mention this since if it pops up it can be pretty flummoxing if you've never seen (heard?) it before.