sorting MP3 and ALAC m4a files in library

I'm at a dead-end with a problem that perhaps others have experienced before...and if not, it's undoubtedly a problem that will be coming soon to many digital music adopters.

My collection lives on a NAS RAID drive, and it's pretty rigorously organized. But it's divided into two separate file types.

The first 'collection' is DRM-free MP3 files ripped at 192kps, which was the highest resolution I could rip at when I started amassing my collection. This is the 'old' collection, dating from the days when drive space was expen$ive.

The second 'collection' is the 'new' stuff, all of which is ripped as m4a (ALAC). This collection started growing a year or so ago, when drive space became nearly free.

The bad news is that all of the files share the same root directory. The good news is that no folder contains co-mingled file types...that is, for example, the folder Todd Rundgren/Something_Anything, contains either 100% mp3s OR 100% m4as.

Now that I'm at the point where I have finally ripped a large enough lossless library, I'd like to find a tool or automated process that will allow me to locate all of the folders containing mp3s in order to remove them.

Doing it manually is just too labor-intensive...there are something close to ten thousand folders to search.

So is there a piece of software, or an automated process, which I could use that I haven't yet found? I'd prefer a mac solution, but pc would be ok, too.

Thanks in advance for any insights you can share.