Apple doesn't sell protected audio-only files anymore (or at least, very, very, very few of them).
Those would be all old ones. You can, if you want, upgrade them to the new DRM-free style, and you get better quality too. It used to be annoying to do so, and they charged a small fee, but with the new iTunes in the Cloud stuff, I think it'd just work. Then you'd have M4As and everything would be right and good with the world.
You'll probably need to delete the file from disk first (or make it so iTunes can't find it anymore -- just move it somewhere safe). Then... You should be able to load up iTunes and go into your purchase history, and download any track you "own" that you've bought from them in the past. I HIGHLY doubt they'd still give you the DRM-laden versions. Plus, you'll get a higher-bitrate copy as a bonus too!
EDIT: Yeah, I just checked it. I never bought from them while they still had DRM, but if you go to the Home Page under the iTunes Store part of iTunes. Over in the right-hand nav "bar" is a link to "Purchased". Click on that, and then you can see all of your past purchases and download them again.
You do NOT need iTunes Match for this. It happens automatically.
Apple still has DRM on their music videos and other videos (the content providers require it), but music should be almost entirely (if not entirely) DRM-free (I think audiobooks might be an exception). For audio-only tracks, M4P is long-gone, and MC never supported FairPlay encrypted M4P video files anyway.
