It sounds like the player is doing something odd since it affects more than one disc. Once you get the DVD playing, can you hit the "Audio" button on your DVD player's remote to see which audio track is in use? Could be useful to find out if the disc has switched from the 5.1 track you selected in the menu to an alternate stereo track, or if the disc is still set to a 5.1 track and the player is doing something screwy with it.