The best you can do is to turn them on mechanically, and then you can control the volume and mute them from the computer. But, there is no A/C power on/off control or power link between a computer and external powered speakers.
I leave my amplifier and speakers ON, and then turn them and the computer on from a power control center. I think that may be what you would like. The Power Center had 5 switched plugs, and you control many peripherals from one place that way.