Combining a silicone rubber case with three miniature speaker drivers that run off of the third-generation iPod nano's power -- lightly, without substantial drain -- rather than an external power source, ifrogz' new Thumpz Audio Wrapz ($25) come in several different styles and multiple colors. The standard version of Thumpz, shown here, has side bumpers and looks like a typical ifrogz rubber case, only larger; a Treadz version uses the company's tire…