Synchronize clips not working in Final Cut Pro

I don't know if this will help in this situation but whenever FCP begins acting strangely an easy thing to try is resetting the preferences. Hold down option and command when launching FCP and select Delete Preferences. It will dump a zip file of diagnostics onto your Desktop which you can trash and then it will open the default Untitled library. Reopen the library you were working on, set the preferences back to the way you like them (turn off background rendering, etc.) and see if the issue has been resolved.

If you double-click the multicam you are taken into the angle editor where you can adjust the timing of each of the angles. Most likely the angle on top is the video clip and will be the "monitoring angle." You can tell because the little monitor and audio icons at the top left of the angle are enabled. The next angle below is the audio only clip. If you select the audio angle then click the little pop-up menu and tell it to Sync selection to Monitoring Angle, does it move it into sync?

Synchronized should be fine for just one video clip with audio and the one audio clip. Sorry to ask an obvious question, but the video clip and the separate audio clip are from the same take, correct?

Just for fun, what happens if you create a multicam with those two clips?

That looks like the inspector for your multicam named Untitled? Instead, select the audio file in the browser and show the inspector for it.

Click the little circled "i" icon to show details of the audio file and post that.

And apologies for asking again, when you listen to the audio of the video clip and the audio of the mp3, they are definitely from the same take?

Are you telling FCP to use audio for synchronization? If so, do all the clips have audio?

If you have multiple video clips and separate audio, why not create a multicam instead?

Can you post a screen shot of the audio inspector for this audio? What was used to record it? When you play it in the browser, does it sound correct?

That seems fine to me. Are you able to create a synchronized or multicam clip with other video and audio clips?