I’m not sure what’s going on. What’s the sample rate of the original file? FCPX (and other professional NLEs) like 48K and 16 or 24 bit wav or aiff files. That might affect t he audio if different. You could try transcoding the phone video to ProRes with Compressor and see if that makes a difference. Also, it’s possible some bit in FCPX has become corrupt. If you know someone with FCPX see if they’ll import and play the file. If no crackling, then you should trash your prefs and even reinstall FCPX. If they hear crackling, too, then it’s something in the file. You might post this question in the Audio Professionals forum. Some of those folks use FCPX.