Could be noise coupled in over ground/power lines of the USB
connection. This is very common, and depends a lot on the particulars
of the hardware set up.
See if you can isolate any other ground connections from the USB
device. Isolate other peripherals from the PC. Test with headphones
for any improvement. Can the USB device be powered independantly from
the USB port?
There are USB galvanic isolators, but they're not all good.
Of course, look at the software mixer as well.
