Audio missing after import

I'm experiencing an issue with FCPX v10.4.6 that just started last night. I've been shooting videos with my iPhone XR for months and then importing them into iPhotos. The clips have audio when viewing them in iPhotos, but 18 of the clips I recorded yesterday (which have audio in iPhotos) are missing the audio track after importing them into FCPX! I haven't changed any settings in FCPX, and I closed the app and rebooted my iMac, but the problem persists. But you know what's really weird? After discovering this issue, I shot a quick video at home, synced it to iPhotos, then imported it into FCPX (my usual workflow) -- and it had audio! I'm at a complete loss as to what the problem is, and I'm hoping someone will be able to help.

Thanks!

Chris