Ok, to avoid arguments with the kind of people who come on pretty strong screaming about 'why you'd want to do this' and 'how Apple's AAC or Loss-less formats are better' let me just take two seconds to explain my situation.
I am running an Apple iMac 27" with a collection of music stemming about 10 years, not massive; about 50GB-ish. Now I have decided to purchase an OEM Car stereo which allows me to play my music from a Micro SD Card! :O I hear you gasp, yes technology is wonderful. My problem of course is, these kinds of players only find and play MP3 formats from these disks... So, now we have that out of the way; I'm going to let you in on a little trick I thought about which will allow you to achieve this.
My criteria for doing it this way, includes:
* The files MUST be in the same folders as the source - I don't want 1000's of mp3 files exported into the same folder so I can no longer identify tracks or Cd's!
* The original files I'd like to keep, for the sake of file sizes and other arguable reasons.
I found you can obviously Change your settings in iTunes, (Preferences, Import Settings, MP3) which then allows you to right click on each song and create an MP3 version. The issue with this is that it duplicates it into the original folder. Thus not getting you very far at all. This will sort your problem though if you don't mind leaving AAC behind. In which case just do a search for .m4a and delete all. You're just left with your MP3's.
You could also, use technology like MediaMonkey or Format Factory again the issue is that these programs output to just one folder!
There is a script file that I found here: http://dougscripts.com/ but I didnt try this out, maybe you may want to give it a go. It looked like it may have worked.
In the end I chose to create a new User, create a new iTunes library; setting the import function to create a copy of the music file and not read straight from source. (File, Library, Organise, Consolidate). So we now have two sets of iTunes music. Now Right click on all tracks, select Create MP3 version and then you now have a full MP3 iTunes library. Finally to finish, go into the new folder, search for all m4a's and delete.
The Final product leaves you with your library left alone in AAC (m4a) format along with a duplicate library containing everything in MP3...
This was what I needed to achieve, maybe someone else needed the same thing... hope it helps...
Did I solve your problem? Buy me a virtual beer by clicking on a Google ad :). Thanks!