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 <http://qjackctl.sourceforge.net/#Configuration>
https://help.ubuntu.com/community/HowToQjackCtlConnections
Hope this helps.
Best
Moshe