Goo! Goo! Soundy is sort of like a do-it-yourself DDR game. You supply the music via your own audio CDs and the game will generate Step charts. You can then dance along with a mat or use a controller.
There appears to be a sort of "virtual pet" style twist added on too. You can pick an avatar who can learn new dance moves and eventually transform into new forms. I haven't really messed around too much since so I can't go further indepth.
As you can see, I suck at DDR, even on the controller. I just could not get the timing down. I'm more at home with the other Bemani games.
Song is Murmur Twins (Guitar Pop Ver.) from the Pop'n Music 10AC/8CS Double Disc Soundtrack.