Can you launch Anki using anki-console.exe (When Anki doesn’t start on Windows (debugging steps)) and post here the output of the console when you try to review a video? This could be related to this old thread: sound playing after 10sec or more
Are you sure you installed the latest alfa version? mpv is only available at alpha2 version or above:
anki-2.1.28alpha3-f44597bb-windows-alternate.exe 09-Jun-2020 19:38 82M
anki-2.1.28alpha3-f44597bb-windows.exe 09-Jun-2020 19:38 102M
