Since I can't seem to be able to edit my post...
After much searching and experimenting, I managed to find a converter. For anyone else who might have the same issues as I:
Download FFmpeg from either one of the following links:
http://www.ffmpeg.org/
http://ffmpeg.zeranoe.com/builds/
Once you have the folder with the program in it, go to the 'bin' subfolder. To make things simpler, you should put all your audio files in here.
Open up the command line and navigate to your ffmpeg bin folder. For those who are unfamiliar with the command line, when you open it up, it should have the active line looking something like: C:\Users\YOURUSERNAME>
If your FFmpeg folder is located on your desktop, type "cd Desktop\ffmpeg\bin"
Note that the ffmpeg folder is likely named something else. Something much longer. You can either rename your folder before doing this, or type the first few letters of the folder, then press tab to auto complete the name.
Now you should be in the bin folder. Suppose you want to convert example.wav into an .mp3 file. Type "ffmpeg -i example.wav newfile.mp3", where 'newfile' is whatever name you want your converted file to be called.
I haven't tried tacking on any other file extensions, since I am perfectly content with the mp3 format, but I think most other extensions should work.
Sorry, forgot to mention that this is for the ADPCM files that you get from the Halo Map Tools. I've tried using the Xbox ADPCM plugin for Winamp but that didn't work...along with a heap of other 'solutions'. So I figured if I can't play them, I'll convert them into something that can be played.
That's really all I came to this site for, so hopefully this info is useful for others.
(Someone might want to merge these posts)