When you click on the Audio tab at the top left of your screen in project view, and click on the Music item in the sidebar, do you get a blank display with the message at the bottom saying that "X number of files are protected and unavailable"? If so, that means that your music is likely Apple Music, rather than iTunes music, that is DRM protected and thus not usable in iMovie.
However, you should have Sound Effects preloaded music (called Jingles) if that is what you are referring to. If you don't have music in the Sound Effects folder, try redownloading 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 and return it to the Applications folder.
-- Rich