Q: Why is the sound different when the project is exported?

We've seen this a few times here on the forum but it has never happened to me. I believe the suggested solution was to delete the clip form your storyline and replace it form the browser; be sure the audio in the browser's reference file is good before you do that. You can export a range from the storyline to test.

You could also try deleting the project's render files.

It might take a few more hours for the wonks to offer other suggestions.

FCPX (and Logic Pro) handle audio internal in 32-bit floating point samples, virtually impossible to clip. So whatever you are listening to when you work in these apps will sound "ok" (terribly loud, I would guess.) But once exported into PCM 24-bit integer format, you will get clipping. You need to pay attention to audio levels inside FCPX while editing. If you're looking at the audio waveforms in the storyline and you see any yellow, orange or red, you should pull the volume bar down until those colors disappear (it's equivalent to the -6dB level on the output audio meters.) The exceptions are: normally loud sounds like bombs/explosions, gunshots, backfires, jackhammers, jet airplanes, etc. However, even those need to be kept at a reasonable level above that 0dB line (if you see a red color with a flat line on top, that will be clipped, no matter what, on export.)

I am curious, Mr Fox, how you deduced mere clipping from this description: "the sound is fine when working in Final Cut, but when I export it, that part becomes loud and distorted." Timeless appears to me to be describing a sudden change in the audio file itself, not a simple level adjustment. You are a wizard of human perception. 

It's simple: if it sounds ok in FCPX (or Logic) and it's clipped on export, the levels are way off. FCPX makes audio editing exceptionally simple because most video editors are not audio engineers. If you see any yellow, orange or red in the waveform, it's too loud. There can be no argument. The 0dB level in the storyline is equivalent to an audio engineer's -6dB level (and the output audio meters are set up to that scale.) All audio meter scales are "relative" to some reference. 0dB is technically the threshold of hearing, but to an audio engineer, 0dB is the loudest a sound should be. There is an extra +6dB that's available for "headroom" before actual clipping occurs. These values correspond to the output meter calibration in FCPX (and Logic).

In FCPX (and Logic), internal audio samples are maintained with floating point accuracy. This allows us as ersatz "engineers" to add compressors, limiters, and other audio filter enhancements without damaging the audio signal (because clipped audio is damaged beyond repair). On export, the default is 24-bit integer sampling. Clipping will occur if the audio is greater than +6dB by the output meters ( — +12dB by the storyline indicator). You can visually tell when audio will be clipped when you see the red "dots" appear over the meter channels (or you see a section of red with a flat line on top in the storyline waveform.)

I am not a wizard of human perception. I am one of those "ersatz engineers" (I've had enough training to know a little ) and have been a musician a lot longer than I've been a video editor (and I make NO claims to be that either... I do have quite a lot of experience using the software though.) I am somewhat "sensitive" to this subject since my first "tape music" project for an Electronic Music course I took back in college was "burned beyond recognition." The effect was kind of cool, but I still got an F on the project (the tape was "so hot" that the signal burned through at least 3 layers of magnetic tape... so I learned the hard way and I know how easy it is to ruin audio.)