But these are songs you don't want matched, and if they're unchecked already then you're probably not copying them over to your devices so none of those things probably matter.
Just out of curiosity, what's the point in having a lot of songs loaded on iTunes that you won't play? I know from experience that the larger the library, the more it slows iTunes to a crawl.
And doing this worked fine for me (for making Match ignore them) without changing types, codes, or file extensions, I just did a get info in iTunes on all the files at once and just changed Music to Audiobook.
Glad it works for you, it was twitchy when I tried relying on it.