Import mp3 into FF6?

You could hypothetically convert an MP3 to a Midi, then that Midi to an MML, and then that MML to FF6 binary. But it's like faxing a fax over and over, the loss would be immense and what you end up with probably would not even be worth the effort.

Maybe mute the game and play your music in another program? Unless you're looking to incorporate it into a hack of your own design, in which case it might be worth going through that fax chain and tweaking accordingly. But it would not be a trivial endeavor.