Reconnecting detached audio to original video clips in Final Cut Pro

To fix once you've done this, select a clip (one at a time), Shift-F will show you the original (with range selected) in the Browser. Now you just do a Replace edit, and you're back where you started, using a clip with its original audio intact.

Correct. What BenB and I describe is replacing your detached video and audio with the original, non-detached clip. The caveat is if you've done a lot of work on the audio and/or the video of the detached pair, replacing with the original clip will overwrite this work. That's why I suggested first duplicating the detached clips so you'd have them as a reference to use after replacing the original detached clips. Depending on the number of clips and the timeline layout, you could duplicate them all or in big batches, then go back and work through replacing with the original clip and reapplying effects, etc., then delete the duplicates as you go.

Once audio is detached, it's a separate clip. Create compounds to rejoin them. Ways to view audio clips in Final Cut Pro for Mac - Apple Support

In addition to what terryb said, and as a note for the future: in many instances you can and probably should Expand Audio, instead of detaching. This keeps the integrity of the video and audio clip as a single entity.

You'll want to copy any effects you've applied first before replacing the clip. You could temporarily duplicate the clip if there has been a lot of work done to it, then use it as a source to copy the effects, etc. to the replaced clip. Once the replaced clip looks and sounds like the temporary duplicate, you can delete the duplicate.

You're very welcome and I'm glad it makes sense. On the plus side, I can't really think of other FCP edit actions that are as time-consuming to undo. I think the documentation should warn that detaching is not a commonly needed action and shouldn't be done without considering the workflow changes down the line.