That DAC/amplifier/speaker "system" was designed for one specific purpose, which is to provide a complete sound system for computers that can send out that specific digital signal via a USB port.
I can't think of any magic bullet that would do that conversion for you.
What Adam suggested will serve to feed a two channel analog signal to this beast and if that's all you want, enjoy bit it's then only a basic computer speaker, and a very expensive one at that.