Can't capture an application's audio when screen-sharing

Chrome only supports audio capture from a TAB or from the DESKTOP. If you would like to capture the audio from a desktop application, use VB Virtual Cable for Windows or Loopback for macOS.

With VB Cable (Free), you can use Windows built-in "mixer" app to push the audio from the application to the virtual cable, and capture that into OBS or OBS.Ninja as needed.

With macOS, Loopback makes capturing audio from an application easy, but it is a paid app.

There are other methods and software alternatives out there to solving this problem.