My system on my Mac 10.14.1 (Mojave) is set up to record as described in the tutorial but the sound wave ‘hugs’ both sides of the line all the time. In past recording sessions (several months back), louder music registered as lots of blue on top of and below the line. But now, after a long time away from recording (and with my memory of those settings not clear), I get louder music to show only a little wider blip along both sides of the line and I have to assume it is being heard by the computer; I can certainly hear it. I’ve just installed version Audacity 2.3.0. (I’ve not played any back to know if sound is being received properly, and lack of a wider wavelength worries me into thinking it isn’t.) Earlier I was recording albums, and now I’m doing cassettes, but I can’t imagine that would make a difference.
You never actually hit what you’re doing. What are you recording, with what and from where? Some microphones and systems are not adjustable in Audacity while recording, some are.
I’m playing cassettes in a deck that’s routed through an amp into my iMac.