Anki stores the sounds and images used in your notes in a folder next to the collection. For more on the folder location, please see the file locations section. When you add media within Anki, either by using the paperclip icon in the editor or by pasting it into a field, Anki will copy it from its original location into the media folder. This makes it easy to back up your collection’s media or move it to another computer.
You can use the Tools>Check Media menu option to scan your notes and media folder. It will generate a report of files in the media folder that are not used by any notes, and media referenced in notes but missing from your media folder. It does not scan question or answer templates, which is why you can’t place media references to fields in the template. If you need a static image or sound on every card, name it with a leading _ (e.g., '_dog.jpg') to tell Anki to ignore it when checking for media. If you delete media using the unused media check, Anki will move it into your operating system’s trash folder, so you can recover if you accidentally delete media that shouldn’t have been deleted.
Anki uses a program called mpv (and mplayer as a fallback) in order to support sounds and videos. A wide variety of file formats are supported, but not all of these formats will work on AnkiWeb and the mobile clients. MP3 audio and MP4 video seems to be the most universally supported.