Wow. What a great variety of fixes!
I had a case that smelled awful too. I tried the bar of soap first, and after a week or so, it still smelled musty-but with a fresh spring scent. I tried baking soda. I tried vinegar. I even tried charcoal odor eaters for shoes. (They actually worked). but the smell was still there a little. I never did try kitty litter.
What worked the best for me, was setting the case outside in the sunshine. I left it out there for hours...moving it with the sun, so that the UV light was always on it. At the end of the day, all the odor was completely gone. It still smells fine now, about 2 years later.