I'm using X3 Studio, and this is quite a different problem, I've never experienced in any version of Sonar I've used.
 
I plug in my mic, using my Scarlett 2i4 interface.  The green halo lights up when I sing, so the gain is set correctly.  With input echo on in Sonar, the meter lights up and I can hear myself fine, with some reverb added in Sonar.
 
Now, when I hit play, the input echo is muted.  The green halo still lights up on the Scarlett, but the meters don't light up in Sonar.  I can't hear myself either.  I can only hear the backing tracks from Sonar.
 
When I hit record, the signal is recorded fine, but again, no input echo.
 
I double-checked all my routing, tried a fresh new file in Sonar, tried restarting Sonar, tried changing the input and then changing it back.  Any thing else I should check?  Anyone ever experienced this before?  

MondoArt
I'm using X3 Studio, and this is quite a different problem, I've never experienced in any version of Sonar I've used.
 
Now, when I hit play, the input echo is muted.  The green halo still lights up on the Scarlett, but the meters don't light up in Sonar.  I can't hear myself either.  I can only hear the backing tracks from Sonar.
 
When I hit record, the signal is recorded fine, but again, no input echo.
 
I double-checked all my routing, tried a fresh new file in Sonar, tried restarting Sonar, tried changing the input and then changing it back.  Any thing else I should check?  Anyone ever experienced this before?  

In Sonar Platinum, there is a setting under preferences|Audio|Playback and recording:

Disable Input Monitoring during Playback.  Is checked by any chance?

Note: I cannot remember if this setting is available in Sonar X3