This allows you to process the audio signal thru MX4's filters, effects, etc.

To route audio into MX4: - Assign an audio track in your project to an available bus (or for stereo audio, pair of busses).

• In MX4, click the enable button to the right of the Audio Input knob.
• Choose your bus input from the pop-up menu under the Audio Input enable button. If you wish to use stereo audio, you must click the "stereo" button in MX4 before choosing your input.
• Turn up the Audio Input knob to the desired level.
• Now as you play your sequence, the audio that is being bussed into MX4 will be processed by the effects, filters, and envelopes. Note that the audio will only be heard when there is a MIDI note being received by MX4 (the amplitude envelope must be triggered for the audio to be heard).

For more details refer to page 52 of your MX4 manual.