It's easy enough to set up Planetside 2 to record gameplay for Twitch streaming (since it's built-in), but I want it to record my voice commentary from my headset microphone as well. It doesn't seem to do this by default, though. Is there a way to do this?
You will need to employ a program that can create a virtual audio card that mixes in both devices (outputs) into one which can then be streamed out.
The program I would recommend is Virtual Audio Cable (VAC). Setting it up may take a bit of reading, but this is what you need to do to get it up and running. I'm certain there are many guides for it!