As per the getUserMedia/getDisplayMedia webRTC spec [see usage], system audio would only be supported on Windows & ChromeOS with no expected support for macOS nor Linux. Screengo would need to become an Electron app and utilize its own loopback audio driver (or pulseaudio) for cross-platform support. Unfortunately I imagine this is outside the scope of this project – but am just speculating.

What could be done easily, at least from a frontend perspective, is add an audio stream of a system microphone – but as per this project's description, would be also out of scope.