Why the error message?
I then restart the program, and everything works fine. But it's obviously wrong in its analysis. First, I had no problems playing songs before the message, so it's not true that MC "could not play" the last few songs. Second, I haven't touched the file pathways -- everything's in My Music on E drive.
What's going on? Seems kind of odd.
EDIT: It works again -- and this time I did not have to restart MC9. What a strange occurence. Maybe it had something to do with the ripping that I was doing at the same time (though this usually is never an issue.)

