What GStreamer plugin am I missing that’s preventing me from seeing Audio/Video properties? As shown in the following pictures:
That depends on the file type of file you are trying to view. For best coverage, you will want to enable all of them.
See GNOME Files: can't show audio/video informations · Issue #53631 · NixOS/nixpkgs · GitHub for the issue and Nautilus Audio/Video Properties: Your GStreamer installation is missing a plug-in. · Issue #195936 · NixOS/nixpkgs · GitHub for a workaround.