If you are downloading them off the net, then there is no need to use DVD Shrink. I'm not sure about DVD Decrypter, because I'm unfamiliar with the program.
The reason that some movies open and others don't most likely has nothing to do with you. It usually means that the sender did something wrong (there are endless possibilities as to what the sender could have done wrong). However, it could be that you are trying to open a file type that is not supported by your media player.
As for the audio and video not matching up, see paragraph two ^^^^^. To fix this (its kinda complicated), you could download a video editor, and realign the sound with the video.
If you can't get all that to work, then I suggest going out and buying the movie.