MP3 Bit Rate Conversion

I recently ripped literally every CD I own to my hard drive using Musicmatch Jukebox....thousands and thousands of songs by hundreds of artists. I was not paying attention and left the quality setting on a wildly unnecessary 320kbps. Musicmatch will allow me to convert them down to 128 (which is where I want them) but I can only do one folder at a time. With the sheer volume of music that I have, this could take days or weeks of work. Does anyone know of a FREEWARE piece of software that can do them all at once without placing each song in the same folder and then having to re-organize them afterward?