MacOS – How to hide a photo in the new Photos.app

If you’re talking about sorting your list of albums alphabetically, go to View → Show Sidebar or press ⌘ cmd ⌥ alt S. Then right-click Albums in your sidebar and choose Sort → By Name.

If you’re asking about sorting the photos inside an album alphabetically, right-click that album in your sidebar and choose Sort → Keep Sorted By Title. Note that most photos won’t have a title, and Photos will next sort by date taken.

I don't know how long this have been the case, but now you can use option ⌥ + Space combination to start playback on a Live Photo or video (works on Catalina)