Thanks everyone! The unit is a stand-alone player. I am located in Mexico (region 4) The discs are bought in the US and Canada (region 1) and also here. When I bought the player, I bought it because I was looking for a player that would play ANYthing - and I tried them all at the store because I had brought back a DVD from Spain. This player was the only one to play it.
This is just a theory I have because it doesn't make sense. The discs, including Chris Rock's raw live performance full of swearing etc, work fine on my Polaroid portable DVD player.
It is interesting what someone mentioned about the chip in the TV - although the DirecTV channels don't present a problem with any sound cutting out. All my equipment, by the way, is run through a Yamaha amp.
Hopefully this clears up some of the doubts people have had. Thanks for all your input so far!
Saludos desde Mexico!