With Focusrite, it's been my experience the track inputs aren't matching the Focusrite I/O routing software setup.
It's possible one can here what's playing if monitoring from the headphone/monitor out or other output even (factory setting is monitor is active) but if the Sonar track input doesn't correspond to an active output from Focusrite, nothing's recorded.
I usually have the Focusrite software open while I'm setting up recording and save a snapshot linked to the project.