I have a similar setup with mixer and all. Forget about using the onboard soundcard, it's not needed in this case. The Yamaha Tyros 5 has a USB TO Host terminal at the back so you can plug one end of a USB cable to this terminal and the other end to a USB terminal on your computer. This directly streams digital data in both directions at the same time. You'll have to download the Yamaha/Steinberg Audio Driver and initiate it in Sonar's Preferences > Audio Devices. Set MIDI Devices to Digital Keyboard and let Sonar be the MIDI timing master. The Tyros itself is used as the Audio Device with its Analog output going to the mixer and onto your speakers so you can monitor what is being recorded. I did demonstrate this technique with a lower level Yamaha arranger Keyboard in the link to the thread below.
 
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