how do you rrecord the midi output from the korg as a audio track?
I'll assume you have a project file with a MIDI track that you can playback on your Korg.
In your project with the MIDI track:
1) Arm an audio track to record.
2) Make sure the audio out(s) from the Korg are connected to your soundcard's input(s). The USB port is for MIDI data only, not audio.
3) Hit record. The MIDI track should play on the Korg and Sonar will record the audio output from the Korg onto the audio track that you armed in step one. When its done, unarm the audio track. You should see the audio waveform in the track.
That's all there is to it.