On macOS with current OBS, "Screen Capture" will include the system audio automatically, while "Window Capture" will not.

Complicated interaction with macOS Sequoia though. I’m on apple silicon and trying to capture on a multimon setup (although only capturing one screen) and the capture freezes painfully often. Going to window capture helps but doesn’t eliminate the problem. Seems to be related to ScreenCaptureKit. I’ve tried a lot to debug it but so far failed.