When creating an Instrument track in Cubase 13, the MIDI Input for the track will default to ‘Not Connected’. If you don’t select a MIDI input, you will not hear sound as you play the keys or pads on your Nektar controller.
You will need to select a MIDI Input in order to hear sound. Select ‘All MIDI Inputs’ (as shown in the image below) or select the MIDI Input for your Nektar controller.
