Hi @Angelo58,
welcome to the forum.
In order to capture the system audio into Cantabile, you will need some kind of ASIO-compatible virtual audio cable.
Cantabile usually exploits an ASIO (Audio Stream Input Output) driver to talk to the audio hardware “directly” (i.e. not going through the system audio). The audio hardware is usually a dedicated audio interface, connected via USB to the PC.
Therefore, you need a software which presents itself as an ASIO driver to Cantabile but it is also capable of accessing the system audio and send it to Cantabile.
An example of such a software is VoiceMeeter. In VoiceMeeter, you have channel strips like in an audio mixer. You can direct the built-in microphone of your PC to one of this strips. Then you can use the “Inserts” of the strip to make the audio go through Cantabile. The setup is illustrated in this video, made by @terrybritton
There are probably other ways to obtain the same result, but this one is the first which came to my mind. Others could probably suggest something better or more up-to-date.
All the best,
Gabriel
P.S. In the video, Terry assigned “A1 ASIO Input Left” to the first strip. In your case you probably need to use “MME Microphone” to capture the audio from your input mic. There are also ways to direct the audio from applications to Voicemeeter (e.g. a youtube video). This probably involves selecting VoiceMeeter as the default audio device in Windows (not sure about this) so that applications will direct their audio output to Voicemeeter.
Edit: see also Terry’s post here: