The traditional way of accessing music is through a USB Flash Drive. But putting them on the SD card is possible. There are some things to consider: There is only a few hundred megs available on the provided 4gb card, and a couple hundred mb may be needed to create temp files during the update process. So off-the-shelf, I wouldn't put more than 100megs of music on the SD card. If you really want to store your than that on your card, you have a couple of options:
1) You could copy all the files from the provided 4gb card to an 8gb card, then you would have tons of room.
2) FOR ADVANCED USERS ONLY: You can delete stuff from the SD card you know you'll never need, IE - the Alaska maps and plates. If you do this, you must delete all the files associated with the item being deleted. IE, "delete \Sectionals\Juneua*.*" (if this syntax doesn't make sense to you, please don't delete files from your SD card ;-).
Thanks,
Walter