Hmmmm. You might click on the loudspeaker icon in the upper right toolbar and make sure that the "lower volume of other clips" is unchecked. The only other thing I can think of is to redownload iMovie. And by all means, turn off automatic updating. It is problematic to be letting anything into your computer unless you specifically authorize it. Sometimes updates can scramble preferences or cause conflicts.
Use this procedure to redownload iMovie. To update/redownload iMovie 10, drag your iMovie 10 app from the Applications
folder into the trash, but do not empty the trash. Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update
iMovie from there. If not successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.
If a redownload doesn't cure it, you might want to contact Apple Support. The techs there have the capability of remotely
accessing your computer that we cannot do here. To contact apple support click on the Get Support item in the upper
right hand corner of this forum’s screen. Navigate to a place where you can enter your phone number.
Good luck with this. It may take some effort, but likely you will be able to resolve it.
-- Rich