Windows only lets you record one source at a time, I'm afraid. There are other bits of software around that get around this by devious routing tricks within the operating system, but Audition doesn't have anything to do with that; it's essentially an ASIO engine. The software I use for recording live streams allows you to add a microphone - that's SoundTap. (I have no connections with them other than as a satisfied customer, incidentally.)
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