Freeware to mix the sound of a file with an toher one:
Here is a free software that give the ability to mix together mp3 files
It is pretty difficult to use and the author ask the register
(i think for free) after a given time:
http://smart-system.pl/mp3/butcher/
Here is a second software with a more pro design but totally free
Be warned that this last software is hard to master:
http://www.kreatives.org/kristal/