I thought of this, then tried something.
I enable a 'volume automation', then dragged the 'end node'; a quick bounce to clips and its done.
If SammyTude does it better then Steph Curry should not have been the NBA's MVP. Win or loose, King James still the better, and so is Sonar...........

All destructive rendering of audio edits will create a new clip; there's no way around that unless you want to give up the ability undo destructive edits or close a project without saving and expect it not to be altered.
 
I know how much you hate workarounds, but maybe this would be more palatable:
 
- Mute the part of the MIDI clip where you want to have silence.
- Bounce with the time range extended as before.
- Unmute the MIDI as necessary.
 
But I'm a little curious about the meaning of "I want to modify that audio 'clip' by inserting a few seconds of silence at the end of it, so that I can use the underlying audio file and not have an abrupt ending."
 
How does adding silence prevent having an abrupt ending?
 
 
 
 

Thanks for the ideas.   I guess I'll get used to the extra render, not that big a deal.  
 
As to your last question, why, brundlefly, it's actually a workaround for another software creation:  the way soundcloud works, anyone who listens to this wip track that ends suddenly will suddenly get started in a soundcloud recommended track ... and if you know how soundcloud works these days, that's a thing to protect your listeners from!    So I always add enough silence to give an alert listener time to hit the off button.  

Ah, okay. I get it. I'm familiar with that phenomenon.

williamcopper
As to your last question, why, brundlefly, it's actually a workaround for another software creation:  the way soundcloud works, anyone who listens to this wip track that ends suddenly will suddenly get started in a soundcloud recommended track ... and if you know how soundcloud works these days, that's a thing to protect your listeners from!    So I always add enough silence to give an alert listener time to hit the off button.  

This Soundcloud thing has been bumming me for some time now. I searched for a way to turn it off within Soundcloud (YouTube lets you turn theirs off), but no dice. It makes me not want to go to the Sonar "Songs" forum. Thank you, William Copper, for caring enough to look for a solution.

That is actually a nice strategy. I was hoping to see a method to shut the "recommended track" off too, but now sort of wondering how much silence you can pad on the end of a song before they trim it.

Here is a way to insert silence in Cakewalk Sonar Artist.   Locate the beginning and end of the section to silence.  Highlight that section like for a cut and paste but don't cut and paste.  Instead got the Process -> Audio Effect -> Audio Effects.  On the "Gain" sliders reduce the gain in that section to 0 (zero).  Bang Zoom section is muted but not gone.