A tiny FCP tip
I had this issue the other day where I had at some point deleted the sound that accompanied a clip from a timeline. The quickest way I found was the ‘match frame’ (shortcut: f) command. Just put your playhead on the start of the clip you want to get the sound back from and hit f. Then the viewer loads with the source clip (not the clip from the timeline) with the appropriate in and out points. Then you can either hit f10 or use your preffered method of inserting thing into your timeline. And there’s your clip with sound. I found this extremely useful when I had to recover the sound from around 100 shots. Then I pressed the down arrow key, hit f, f10 and repeat.