Sonar will stop automatically in playback mode when it sees no more data or audio past a certain point.
 
If a track has a series of dots down the center of it, that indicates there is something further to the right. Sonar will play to that point. This seems to happen most often when I import audio and midi. If I create a new track and record audio to it, that mysterious stuff to the right never occurs.
 
I have had a few tunes that had silence about as long as the song itself..... somehow, something was placed well tot he right in one track.... look for the dots........................................................... follow them and you'll find it.
 
I always export my files to a certain folder on the desktop and then I re-open them with a WAVE  EDITOR from NCH called WavePad. Its free. I use it to trim the count in silence and also to trim the end of the song. Every song I export has silence in both locations and I want the song to start less then 1 second from the start and stop when the last reverb trail has faded sufficiently so it doesn't sound chopped off.

I long ago gave up on relying upon SONAR to figure out where the song ends. Instead, I insert "start" and "end" markers and use them to set my selection (navigate to the "start" marker and press F9, then go to the "end" marker and press F10). This eliminates the need for subsequent trimming and makes every export consistent. And I never have to go on an event hunt looking for that renegade automation node or MIDI event.
 
What SONAR really needs are coded "start" and "end" markers that define the song's start and end points. These could easily be incorporated into the existing markers feature by adding two new properties to marker objects. Special markers would be drawn in a different color. When SONAR calculates the song's end point it would simply look to see if an end-marker exists and use that. I've been making this suggestion for years. Noel called it a "good idea". In the old days it's the kind of enhancement that would have quickly found its way into the product, but this ain't your father's Cakewalk anymore.

Why is that suggestions that "make sense" never get incorporated into anything???
Inquiring minds want to know!
 
Just sayin'...
 
T