Can you run anki from the source code? It should be easier to change things and try them out. For example, I run Anki directly from the source code and patched the sound button from return f"[sound:{html.escape(fname, quote=False)}]" to return f'<audio src="{html.escape(fname, quote=False)}" controlslist="nodownload" controls=""></audio>'.
I think a new button to add media as an HTML audio element should be enough to have specified media not playing out automatically. In my decks, I only use javascript and HTML media, with an addon I wrote: HTML audio/video tags support addon (support thread), it integrates HTML media with anki. Still, it would not work for Android or Anki Web unless I embed the addon javascript in my card template (which I will do if I need to study outside Anki Desktop).