Hi mate, I've started using mine recently and had the opposite problem - I was wanting to send the microphone down one channel but not the other (for live streaming and recording).
To switch these settings you:
1. Hold down MENU (UTILITY)
2. Scroll to Microphone setting - it should give you the options for either MASTER OUTPUT or BOOTH OUTPUT or BOTH
This is from memory, so the wording may slightly differ but you'll get the idea when you find it in the menu. You change the setting by clicking into it with the black circular knob to the right of the screen, and circling it around til you get the setting you want.