When you add a game, make sure to check the setting, "Capture Audio (Beta)." It will add the game/program's audio as a separate track. That way, you can mute your desktop audio, and only have your microphone and game audio playing.

Just watched a video on it haha I'll poke around in the settings, looks like you have to do it for every new game you play

Yeah, you have to add it manually for each game at the moment.