That's a good feature request, but there are ways to do that now with some outside help.
If you're on Mac, use an application like Soundflower or others. For Windows there's JACK Audio Connection Kit and Virtual Audio Cable.
If you have Camtasia, Screenflow, OBS, or any of many other types of screen recorders, you can record your screen--including audio--and then open the MP4 in Audition or export just the audio from the recording to a WAV, MP3, AIFF, or what-have-you, without dealing with a
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