Hello,
When I insert any soft synth into a midi track, I don't hear anything. I noticed that when I insert the soft synth, the icon does not change to a soft synth but stays a midi icon. 

Soft synths are inserted into audio tracks as this is where the audio output from the VSTi (softsynth) goes when the MIDi data on the MIDI track plays.

Track icons need to be manually changed. By default New MIDI and Audio tracks have their respective icons but there is a vast array of icons to choose from. Right click icon and make your choice.

It is easiest to insert a soft synth from the browser/synth rack as it will automatically route the synth for you (and can choose more options prior to the insert). If you want to send a specific existing MIDI tack to a soft synth, this is also the best path... then can either route the existing MIDI track output to the soft synth or copy/paste the MIDI data (holding shift while dragging the data into the synth track will keep it from shifting in time on you, and is quickest).
 
http://www.cakewalk.com/Documentation?product=SONAR&language=3&help=SoftSynths.04.html
 
FYI - the Track View defaults to "Custom" on the drop down in the upper left. I always set that to "All" then I can expand the left pane to easily see input (I) and output (O) assignments.

I have other projects with soft synths inserted and they play fine. The most recent project I have inserted a soft synth into does not. When I play my midi track through my soundcard, it's fine, but when I insert the soft synth, no sound. I can play my keyboard through my inserted soft synth in my other projects and see it playing onscreen and hear it. But my most recent project I can hear my keyboard playing but there is no reaction onscreen at all. This project and my other projects are midi tracks with the soft synths inserted. 

Soft synths need MIDI data. Soft synths alone contain No Sound.
 

 When I play my midi track through my soundcard

Because you are using a non professional sound card that has sound banks in it. Let me guess, a sound blaster card?
 
Do the tutorials for Sonar 1 and Soft synths and you will learn how to use them
 
CJ