Merge audio files and export to browser

What I have: Use can add sounds to the `drop-into-`box` component, and user can now able to play the sounds simultaneously, using [login to view URL] library, by pressing the play button in the blue box. What I need: I want the user, once he adds at least one sound to the `drop-into-`box`, to be able to export his new mix of those multiple sounds, as a .wav or a .mp3, directly to the browser. For example, if the user clicks Add Sound on "/audio/[login to view URL]" and also adds "/audio/[login to view URL]", then he should be able to export a new mix of those two sounds to his browser- exactly the same mix as when he clicks the top play button to the left of the export button. I believe ffmpeg is the best server-side library to handle audio merge, but I am not positive. I also think that this audio-merge needs to be done server-side, not client-side.