It would seem like the Default "Open with" option is Sound Studio for .mp4 files. Here is a way to change it:
http://www.howtogeek.com/howto/4253...ault-application-for-a-file-type-in-mac-os-x/
If you also click Change All, that will change it for
all .mp4 files (once you confirm it).
I just did that on Season 1 of the TV Series The Americans. All the .mkc files originally would be opened with DivX Player. But, I changed one of them to open with VLC, clicked Change All, got a dialogue asking me to confirm that I wanted the change to apply to
all .mkc files, and clicked OK. It worked perfectly. (I also did it with a .mp4 file for Season 2 of that same series, and it worked).