Apparently, there is ImmersionKit API (with only one endpoint). However, it isn’t said on the website. I look into that UserScript’s source code instead.
About pulling from YouTube transcript, it is possible too; but actually I won’t do it yet, as it doesn’t seem to worth the effort that much. I will use Microsoft Azure TTS for now.
Actually, as of now, my Anki card is “EN → JP + autoplay sentence audio” and “JP → EN + autoplay vocab audio”, so yeah, I do need sentence with audio, if possible, for every vocabularies.
I also realized about this recently, due to Jotoba - A new alternative to jisho!