Greasy Fork is available in English.
A player designed for 4chan sounds threads.
< 4chan sounds playerについてのフィードバック
Theoretically yes, but in practice you're better off doing it yourself with ffmpeg. To do it in browser would be using a js version of ffmpeg and it doesn't work great.
I wonder if it'd be possible to add a feature to combine and export the audio and video into a single video file?