Audio Track Programming - Combine 2 Audio Files

I am looking for a developer who can write the code needed to combine 2 audio tracks into one. The input and output will be in MP3 format.

This is not a website project. I have java developers who have already built the website. It is new functionality to be added to the website.

The functionality will allow for the user to select one audio track from a list of prerecorded sounds (mp3 files). Whichever of these is chosen will be the dominant (or audible) sound. Then the user selects another audio track from the song database. When the user clicks submit, the 2 songs get combined into 1 audio track (mp3).

The combined track is going to be the same for both ears. But the thing that will be really important is that we need to allow the user to adjust the volume of the separate tracks either up or down.

Each user will have their own individual preferences in terms of how audible the subliminal portion will be. When the 2 tracks are combined, I would imagine that we have a lower default volume setting for Track 2, but then the user would adjust the volume of each and once they are both at the proper level, then the new combined track could be saved to the database.

Please let me know if you have any questions.