If you are wanting to use the Streamer feature in Sonar when using your Xsplit follow the steps below,
NOTE: This mix will transfer over from Sonar to Xsplit, (this will include any levels and configurations you made in your streaming mode) along with your mic sound settings as well.
Hop into Xsplit, click on the audio settings icon. Then click on the device settings button.
Then set "Steelseries Sonar - Stream (SteelSeries Sonar Virtual Audio Device Stream)' as your system sound. *For your mic you can set it to none as the Sonar Stream will contain your Mic settings.
Set the Audio Mixing Sample Rate to 48 kHz.
Then you are all set!