I can't figure out which additioal software is needed for downloaded movies that won't play on my OSX 10.6.8. I presume I have all the automatic updates but it will only play mp4s. For .avi files I get "movie can't be opened. May need to install additional software" OR "movie not in a format that Quick Time Player understands". For .mkv files I get "Error Dolby Audio, player does not recognise the audio in this video". My TV has no problem playing all of these files. Any ideas?