Overlay (mix) two mp3s into one - upload form - repost

I need someone to customize the [login to view URL] song upload form (PHP Jamroom 4). When a user uploads an mp3 file and submits the form, a second vocal mp3 file will overlay the user file. Both audio files will be mixed together and saved as the user uploaded file. It is also very important that the mixed file have good audio quality. I'm pretty sure that the audio processing can be done using sox or ffmpeg. Repost Attempt #2 In case of any misunderstandings, you will need to customize the Jamroom 4 song upload form ([login to view URL]). Please do not bid if you just want to send the FFMPEG code. Your job will be to login to a sandbox version of the website and customize the scripts (ftp) so that this feature works when I upload a file to the server.