Comment 8 for bug 1280794

I believe the bug here is that once you use System Settings > Details > Default applications for audio or video you then cannot individually change any related mime in nautilus > properties of a a/v filetype to a new default.

Test case:
delete ~/.local/share/applications/mimeapps.list
Open system settings > Details > default apps.. & set the default for either audio or video to some other player
(Ex. video to vlc
Then right click on a related filetype > properties > open with & try to set to another player, it will not do so
(ex. r. click on a .mp4, try to set to Videos