Can you just buy another USB extension cord? How much more length do you need? Those cords can be pretty cheap.
I'll bet that if you ran the power seperately from the USB hub itself (i.e. run just a pair of wires back to your computer and splice into a +5V lead, or the keyboard jack, or the other USB jack) it would get rid of the noise. The sound has to get to the USB soundcard via digital means anyway, so it can't be affecting the signal wires, so it must be coming in via the power wires.
Alternatively you could wire an inductor inline with the USB power wires (again though you'd have a splice a cable)... That might help get rid of the AC fluctuations in the power that's likely causing the noise.