If Plus isn't supporting an MP3(Do what Treylina and Stijn are saying in the above posts first, before concluding that it's Plus, though!), that's possibly a bug with Plus -- take it up with the Plus devs(But again, only if you're 100% sure it's Plus that's broken).

As for the conversion, not sure why that's not working... Also, Stijn's right, converting to a WAV for conversion will create HUGE files. Try as many different formats as you can, though. Maybe you'll find one that works.