Does 3DS need an SD card to work?
No. Though functionality, like StreetPass and Nintendogs' in-game photography, won't work without one. The machine comes with a 2GB card, which should tide you over. Unless you plan on taking 3,000 dog pics. You... don't, do you?
What happens when I stick a 3DS SD card in my PC?
You get a lot of digital gobbledygook (save files) and some JPGs. Every 3D photo (saved in an MPO format) is cloned as a JPG. If you're feeling particularly mean you can stick these back into a DSi to show it what it's missing out on.
Anything to say about the Sound Recorder?
Not really. It hasn't changed a great deal since DSi. It'll play MP3s as well as AAC, which is handy. Oh, and the visualisers are in 3D. This spaceship number shows us how cool Rez would look in 3D. *Dreams of 3D Rez*.
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