Join (video/audio) in c# coding via mixing

Since you want to Join (or separate) video and audio streams using ASP.NET web application, you can create a service (at the server side), pass the Media files to it from the client side, and then separate or Join the Media files as needed using DirectShow functions and filters. I found the following links that might help:
http://stackoverflow.com/questions/2694793/how-to-implement-video-and-audio-merger-program
http://superuser.com/questions/277642/how-to-merge-audio-and-video-file-in-ffmpeg
https://www.leadtools.com/blog/multimedia-imaging/how-to-combine-video-audio-separate-sources/