The wave shield uses SD/MMC cards. They are extraordinarily popular, sometimes even available in grocery stores! They are used in MP3 players, cameras, audio recorders, etc. You can use any card that can store 32 MB ore more. A 4 gigabyte card can hold 25 hours of uncompressed audio for the shield, and costs $12
The shield kit doesn't come with an SD card but we carry one in the shop that is guaranteed to work. Pretty much any SD card should work but be aware that some cheap cards are 'fakes' and can cause headaches.
You'll also need a way to read and write from the SD card. Sometimes you can use your camera and MP3 player - when its plugged in you will be able to see it as a disk. Or you may need an SD card reader
These are very common, available in any computer store. The shield doesnt have the ability to display the SD card as a 'hard disk' like some MP3 players or games, the Arduino does not have the hardware for that, so you will need an external reader!
To use the SD card interface library you'll need a '328 Arduino. If you have an NG or '168 chipped Arduino we suggest upgrading to a '328. Its easy and inexpensive and you'll be very happy with the 2x RAM and Flash. All UNO's have Atmega328 chips
If you bought an SD card, chances are its already pre-formatted with a FAT filesystem. However you may have problems with how the factory formats the card, or if its an old cart it needs to be reformatted. The Arduino SD library we use supports both FAT16 and FAT32 filesystems. If you have a very small SD card, say 8-32 Megabytes you might find it is formatted FAT12 which isnt supported. You'll have to reformat these card. Either way, its always good idea to format the card before using, even if its new! Note that formatting will erase the card so save anything you want first
We strongly recommend you use the official SD card formatter utility - written by the SD association it solves many problems that come with bad formatting!
Download it and run it on your computer, there's also a manual linked from that page for use