This is probably a long shot ,but thought it might be worth a mention.
Several posts in FCPX have talked of import problems since High Sierra 10.13.2 .
One or two concerning no audio when importing AVCHD and others with importing Mini DV in general.
The advice has been to uninstall Perian and A52.
I don't know much about these , only what I read in the posts.
The following is copy/paste of sections from 2 posts.
Luis Sequeira1Dec 21, 2017 3:40 AM in response to Mickeyfilm
Level 6 (19,586 points) 
Mac OS X
Dec 21, 2017 3:40 AM Re: When I import a .mts files, also known as AVCHD, only the picture appears, no sound.in response to Mickeyfilm
To uninstall Perian, I think you have two things to delete: the preference pane, and the Quicktime plugin.
And then RESTART your mac!
The preference pane is inside your Library folder: ~/Library/PreferencePanes
The Quicktime plugin is either in ~/Library/Quicktime or /Library/Quicktime.
Don't forget to RESTART.
Luis Sequeira1Jan 25, 2018 10:05 AM in response to ericalex47
Level 6 (19,586 points) 
Mac OS X
Jan 25, 2018 10:05 AM Re: Import of Sony Avchd filesin response to ericalex47
Do you have Perian or A52 installed?
You may not realize it, as it may have been there for years.
Perian would appear in System Preferences, in the last row.
The A52 codec may be present in /Library/Quicktime or ~/Library/Quicktime.
Both of these are known to cause sound disappearing in AVCHD material.
Some posts say that removing Perian has cured their fault whilst others say that it hasn't, so this may not work for you, but worth a look to see if you have Perian or A52 .