Ah, I didn't realise that was an output.
I was trying to hear the VCO without anything else, as per your first step (I hadn't gotten to the second step yet). However, even with the AI (Focusrite 4i4) gain at zero, it still came in too hot. Hence, I ran it through the mixer first, as someone suggested to me before. I stopped at that point, as the buzzing was clearly a problem, and I didn't want to push on until it was rectified.
I am connecting to the AI via a 1/8 mono cable with a stereo 1/4 adapter on the end - it has been a drama getting a 1/8 to 1/4 mono cable from my supplier. Might that be part/all of my buzzing problem? I am not used to using 1/8 mono... This is connected into the front 1/8 jack with the adjustable gain.
I will ignore the buzzing for now, and push on with what you wrote before (and not plug outputs into outputs...)