I am trying to change some mkv (with ac3 5.1) files to m4v without re-encoding, and I chose to use the subler route. All of the files I have tried are over 4GB in size (I checked the 64-bit option, since that's what I have read). First of all, after I import the mkv file to subler, I can't select the optimize option. What would cause this? I just selected the save option and chose 64-bit and clicked save. Only one of the files I tried has worked. All of the others end up unusable when the saving is done. I don't know what to do or why this is happening because they are all mkv files with ac3 5.1 audio, and one worked when the others did not.