I don't have a setup in front of me to try this, but Sonar should be set to receive MIDI clock if that's what Traktor is sending. MTC and MIDI clock are not the same thing.
From Sonar's documentation:
1. Configure the external MIDI device you want to use as the master device to transmit MIDI Sync.
2. Go to Edit > Preferences > MIDI Devices.
3. Make sure that your MIDI interface is highlighted in the Inputs list, and click OK.
4. Click the Transmit MIDI Sync button in the Sync module to use the MIDI Sync clock source. From now on, SONAR starts playback and recording only after the appropriate message is received from the master device.
It seems counter-intuitive to click transmit MIDI sync to receive MIDI sync, but I guess that's how it works...