用于覆盖MP3文件以在PHP中使用的Linux工具

I have been searching the internet with no success for a tool that will allow two mp3 files to be overlaid on top of each other. I have a client who wants to build a service that allows someone to select a music track and a vocal track from a list of files and then have them play together. Her business produces visualisation CD's and I believe this would be the best route as she has hundreds of visualisation mp3 scripts and about 30 backing music. This will allow the merging of these 2 files dynamically and for them to be cached as opposed to doing the manual work on demand.I have been searching the internet with no succ