Digital audio player, help choosing the right device

Ehm, it's kinda awkward to say this on an Altera forum, but it doesn't look like you need an FPGA here. Reading a FAT filesystem means that you probably need a decent processor (most likely some ARM, but there are other around). Audio is something commonly supported. You can, of course, do this with an FPGA or a SoC device, but it's a heavy overkill, and odds are that a plain processor device will be the most cost-effective.