Adding audio to a video

I am trying to do a website which will record your video, remove the audio and add an audio to the uploaded video.

I did the recording (without audio) using html5, and I uploaded the video to the server, so now I have video without audio, but what I cant figure, is how to add the audio to this video.

Please notice, all the videos uploaded are 10 seconds, and the audio also 10 seconds.