Can you please post your code (using code tags) and a schematic showing what you are trying to do? It seems like you're having trouble using a Mega to play audio using SimpleSDAudio? Going by the information here it should definitely be possible to use a Mega rather than a Uno.
Using a MicroSD rather than a fullsize SD shouldn't make any difference. I haven't used SimpleSDAudio but I have used TMRPCM with a microSD card before.