Merge Mp3 audio to and an mp4(only video, no sound) files to get 1 mp4 file with audio in C#

hello,

i am recording audio and video in 2 separate application and i get 2 different files 1 mp3 for audio and 1 mp4 for video.
i need to play just 1 mp4 file that contains the audio mp3 .
solution should support multi threading to handle merging multiple parallel files