>I don't know what you mean by opening iMovie in a "new library".
Open in a new library by holding down the Option key while opening iMovie. You will get a dialogue box that allows you to select the library in which you want to open, including creating a new library. Select to create a new library and iMovie will open in a new blank library. Create a new project in that library and load in your troublesome video clip. See if the sound plays at full volume there. You won't see your old projects in the new library. To get back to your projects in the old library open it by clicking on its icon in the Movies folder on your Mac (Go/Home/Movies). The purpose of opening in a new library is to rule out corruption in your old library.
>If I re-download iMovie, will that get rid of all my files I already have in there? (since I assume re-downloading means first deleting my current iMovie?
No, your files are kept in the iMovie Library, not the app. When you redownload your iMovie app, it will read your old library and all of your projects will be intact and no media will be lost. So don't delete the old library -- just the app. Here's the procedure:
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 and return it to the Applications folder.
There are other diagnostic procedures that you can try as well, such as the following:
Try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects, as described earlier in this response. Now see if the volume plays properly. The purpose of this harmless procedure is to reset any preferences that may be corrupted and causing iMovie to misbehave.
You could try booting up in SafeMode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly. Don't try to open iMovie while in Safe Mode, because it won't. The purpose of this procedure is to clear out any conflicts that might be causing your issue.
Another thing that you can try is to create a new user account (System Preferences/Users and Groups) and see if iMovie will work properly in the new user account.
Another thing is to reset your System Management Controller. It is easy to do. Here's the link with instructions:
https://support.apple.com/en-us/HT201295
Hopefully one of the above will isolate and/or cure the sound issue.
-- Rich
