I've just spent awhile looking at similar complaints from all over the place. It was a google decision to no longer support WMA files. Si it's not a Kobo specific issue, or a Vox specific deficiency. Folks are pretty angry, that's for sure. I gather that some folks had 100s of WMA files, and ended up having to rip them etc if they wanted to continue to use them.
It seems to be about licensing of Android and Microsoft fees for using the format. From that POV, it is akin to Kobo having to pay about 22 cents to Adobe every time we download an ADE ePUB from their store.