When I switch my oscillating floor fan on it turns the audio off on my PC
Most likely the fan switch sparks when turning if off, and it sends a burst of electromagnetic interference. That should not happen. But the USB audio interface should handle the interference, so this should not happen. Either the fan emits more than allowed, or the USB device tolerates less than required. Try changing the USB cable, but also other cables can act as an antenna so reorient the devices and cables.