Hi Mark,
Thanks for reaching out!
There is currently no option to change the audio input of Storyline 360's screen recording to the system audio. If you're using screen recording to capture an online video, a better alternative would be asking for a copy of the video instead, and editing it in Replay 360.
Having a system audio input available for screen recording sounds like a great feature to have. I’ll be happy to pass this on to our product team, but you can also feel free to detail them more through a feature request!