The installation of the driver software may be incomplete or the connection may be causing a problem.
If you’ve already installed the driver software, try again with the following procedure:
1. Close rekordbox and disconnect the unit from your computer.
2. Open the control panel (or [Start] > [Settings] > [Apps]) and uninstall [DJM-A9].
3. Close Windows.
4. Disconnect other USB devices, USB hubs, etc.
5. Start Windows.
6. Download the audio driver software from [here].
7. Install the audio driver software again.
*An error will occur when the license agreement screen is displayed if you try to proceed without clicking "I agree".
8. Connect the unit to another port without using a USB hub, etc.
If you still can't set [AUDIO], the problem may be caused by operating system settings, etc.
Refer to cases 3 and 6 in the guide [here].
Follow the instructions in the guide [here].
If you still experience the problem, it might be caused by operating system sound settings, etc.
Remove all other USB devices from your Mac, uninstall the virtual audio device etc, and restart the Mac.