Stop using your computer immediately! The longer you use it the lower your chances of recovering your files.
The files are now free space on your hard drive. Any new files created will over-write free space, and likely destroy the last records of your files.
Turn off the computer immediately. Download a Linux Live CD (e.g. Ubuntu Linux) insert it into your computer and install the PhotoRec tool which you can get info about here: http://www.cgsecurity.org/wiki/PhotoRec
It's not just for photos. It's a free tool that can scan your hard disk for deleted files of various types.
Since this post isn't about Mixxx, I'm moving it to the Mixxxcelaneous section.