aunkster... for what it's worth, I had the same issue and never got a clear answer.
I emailed a buddy who has a 8830 a ring tone and it worked fine. I emailed myself the same ring tone, could save it to the card, but it would not play. Loaded the same mp3 onto the card via the card reader and it would play. Re-emailed the exact same file to my BB, saved it, it would not play. So I gave up and just copied it my card with a card reader. And it was only one particular ring tone. I tried several right after via email and they worked. Who knows!