I am trying to place audio into the "Background Music Well," which is the last track in the timeline, at the very bottom. It's marked with music notes at the start of it, and has a thin black border. This track locks the audio in place and keeps it separate from any changes made to the audio and video clips above it. It allows you to edit picture to sound, like for a music video where the audio must not change. There is only one Background Music track.
Normally you just drag an audio clip into the Well and it sticks there. Then any edits you make to the video or audio above it don't affect what's in the Well, and vice versa. And normally I don't have any problem doing this, so this is a glitch of some kind. I can successfully place any clip I want into the usual audio tracks, just not into the Well. There are no DRM issues involved.
This morning I restarted the Mac while resetting the NVRAM (pressing option+command+R+P). The problem persisted. Then, wouldn't you know it, thirty minutes later I came back and tried it again and it worked the way it should. So... resetting the NVRAM did the trick? Or not? It seems to be behaving now, maybe the gremlins are on coffee break, but I'll get back to work. I'll leave the thread open if you have any further insight or comments. Thank you for your interest.