Combining audio and video on the fly?

Question

PHP

Client video.js
Server FFMPEG

The server has the original video without sound and its other formats in chunks. Audio is encoded according to Youtube standards

While I am thinking about how to properly organize, combine on the client, then it will be necessary to stream audio and video separately on the client and load its device by combining audio with video on the client, or implement it on the back end and stream the already combined video with audio .
An example of a response to CO , but I'm not sure if this approach is correct.

Need advice on which approach is better and why?
What libraries or packages should be used for combination?