i use tversity also.
as you have stated some movies will play and some not.
sound on some movies and some not.
i also have this problem.
depending on where you d/load them from
look to see if they play on ps3 and xbox.
i usually d/load from the same person and have no problem.
its to do with the formats xvid/divx/mp4 formats and i have no idea why some play and some dont.
i have had the same films with the same ripped codecs and some play and some not play
when you investigated further you find some are divx and some xvid and then the
sound codecs come into play
stick with a person's rips you know works.
blue ray up to 2 gb are fantastic.
stay away from movies that are joined half way. you find that you have to restart the movie to play the second half.
.
its a good piece of kit but does have its quirks.
i find most mk4 files are good.