It must just be a bug somewhere in the definitions for a Mega. Since the example libraries only support MMC I decided that rather than debug them I’d just adapt a library that supports full SD, SDHC and SDXC. Rodan’s playa library works well, and I finally get to hear my shield working!
Link:
https://github.com/rodan/playa
P.S. When I finish adapting it to the Seeedstudio Music Shield I’ll post the code for anyone else who has problems with Mega’s and this shield