I've seen this issue pop up before, and I'm not entirely sure how to fix it, but if the file type isn't the issue, you may have to find another way. Where have you gotten your music from? A converter? Did you buy them? That kind of stuff.
Hmm. The one time I saw this issue pop up for someone else, they got it to work by using a different converter, so I am unsure why it wouldn't work with a normal MP3. I suggest trying to report this in the pinned bug thread. Maybe somebody can help you there? Sorry I couldn't help more.
