The audio signal from a device connected to the MIX IN jack of the J-6 is not sent to the computer through the USB port. To record this audio, use a cable with a stereo mini phone type plug to connect the MIX OUT/PHONES jack to an audio interface connected to the computer. Note that the MIX OUT signal is a mix of the MIX IN audio and the J-6's internal sound module audio, not the MIX IN audio alone. For more on the signal flow, refer to "Signal Flow" in the J-6 Owner's Manual.
Note: The audio signal from a device connected to the MIX IN jack is not routed through the USB port to a computer. Only the internal sounds of the J-6 are routed through the USB port.