I sincerely think it is these two disks, that have their exclusive and individual problems when I play either one of them, as any other DVD that I put into the player works just fine, sound as well as picture! Somehow my remote has turned off the sound and turned on some kind of background muzak-like music-only channel (with picture okay!) Actually, the sound is there, but it is music instead of the actual audio from the DVD!
And apparently it is on the disk, as the problem with the other disk (which is different from the first disk, as noted, i.e., sound, but music instead of DVD audio), seems to have likewise been caused by some other user/renter having done the same thing I did on the first disk, which was to hit some wrong combination of buttons, completely turning off the sound entirely! As I say, I have rented this very title previously, and the disk that I rented had sound! Of course, it is not guaranteed that this specific rental disk is the exact same one that I had rented previously! But the problem is now the same as with my first disk (my own, not a rental!) The rental disk that I received this time has no sound/audio whatsoever, so some previous user has apparently done the same thing that I inadvertedly had done on my disk... hit the wrong buttons on the remote, turning off completely the sound/audio!