If you look at the Adafruit website, it says the wave shield is NOT compatible with the Mega (only AT'328 chipsets) so that won't work.
After you change boards, what you will need to do is read in the song list and store it in an array rather than print it out.
Also, you button checking seems very complicated and not really needing to be done inside an ISR. Why don't you just check at the beginning of the loop()? There are also libraries for buttons that do all the debounce sort of stuff.