I had a problem with audio dropping out at the end of each clip in a FCP timeline. The waveform was still showing, but the clip was muted.
I found that the problem was a third party effect from iZotope RX 7. Removing these effects restored the audio.
And then I was able to export the timeline, bring it into RX 7 and apply the effects, then bring the wav file back into FCP before exporting the video file.
I hope that helps someone!