You can fix this in 3 steps.
- In the Sources box in the lower left corner, click "+" and select Audio Input Capture.
- In the pop-up box, under Add Existing, select Mic/Aux. If this is not an option, select Create New and click OK.
- Then, under Device, select Built-in Microphone or your preferred microphone and click OK.
You can run a test recording on OBS to make sure audio records both your microphone and Minecraft game noises. If you don’t want to record Minecraft sounds you can mute the noises within the game preferences.