IMHO the WTV020 modules if more trouble than its worth.
You can't tell it to play individual tracks, all you can do is press play, or stop and next and previous, which makes track selection a pain (but probably not completely impossible)
In addition, the module doesn't play real MP3's it has to have specially formatted sounds
And... It doesn't work with all SD cards
I'd look at a different module, possibly one from Adafruit or Sparkfun which has proper controls.
PS. I've also tried controlling those cheap iPod clone players you can get on eBay, and they are not much better, but at least they play MP3's
(but they still only have play, pause, next and previous etc, so track selection is a pain / useless)