I’m pretty sure that grabbing only the audio stream from a YouTube video (especially without a subscription) is a bit of a grey area at least (or it might break ToS, not sure). YT-DLP does it, NewPipe on Android does it (it allows video/audio downloads, playing with locked screen, etc), but I’m not aware of a solution that does it in a browser with purely client side JS. Then again, if you’re making an Android app, there’s probably a way to hack together a Cordova plugin that grabs the audio stream and passes the URL to the JS runtime. NewPipe is open source, so you can probably check out how it does it.