Proof of concept video player for Mac OS 9 that uses modern libav (used in ffmpeg and VLC) and supports more modern formats. Why? We don't ask those questions.
Only does video, and has a known bug where mp4s don't play (but other video formats with the same codec do play) on weaker machines. There was an attempt to get audio working, but I spent literal days on it and eventually gave up because anything that ffmpeg will play nicely with is either too new or too hard.
To download, see the releases sidebar. To build from source, just get Retro68 and use ./build.sh.