I'm running Sonar Artist, and I had the same issue with Sonar not stopping at the end of a track.
I called Cakewalk Support directly and got this
Fix for the problem.
The Fix:
In the
Track View Pane choose
Options (located along the top of the frame around the track) A dialog box will open. Go to the bottom of that dialog box and check "Stop at Project End", click outside of the box to dismiss it. This will save the setting. (And it will stay checked from project to project.)
I suggested to the person I spoke with that
this should be checked by default. Users of older versions of Sonar and other versions
expect "Stop at Project End" to be the default behavior. That way a user could uncheck "Stop at Project End" if they wanted the playback to continue past the track.
This works great in Artist, I don't know about the Sonar Platinum or Professional versions. I assume the dialog box would be in the same location.