Um ... I'm not certain what you mean. Normally, Audacity
routes its output signals to the PC's speakers. What I want to do is
to route signals that are normally headed for those speakers
to go to the Audacity input ... I am not an expert in JACK;
I wish I was, but to me it is mainly a behind-the-scenes resource.
:-)

If you've got some sort of Jack front end (like qjackctl) you could open the "Connections" window and route audio signals where you want them.

Some info:
qjackctl

https://help.ubuntu.com/community/HowToQjackCtlConnections

Hope this helps.

Best 

Moshe