From the help file of Sonar 6 PE
Choosing Clock Sources: SONAR as Master
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When SONAR is used alone, or with an external device that does not have its own clock or timing signal, you use one of two clock sources: Internal or Audio.
When the clock source is set to Internal, SONAR uses the clock built into the computer or the computer's MIDI interface as its timing source. If your projects contain only MIDI (no audio), this is the most efficient method of playback. If your projects contain MIDI and audio, or only audio, you should set the clock source to Audio. This lets the sound card clock determine the correct speed for audio playback and automatically synchronizes MIDI playback to match the audio. For more information, see System Configuration. You cannot use the tempo ratio controls when using the audio clock, because the audio playback speed is determined by the audio clock.
When either of these clock sources is used, you can also configure SONAR to drive other MIDI devices using MIDI Synchronization. For more information, see MIDI Synchronization. If you need to send MIDI Time Code (MTC), SONAR will send this data regardless of the clock setting.
To Use the Internal Clock Source
Click on the Sync toolbar.
OR
Choose Options-Project, and click the Clock tab.
Check the Internal box.
Click OK.
To Use the Audio Clock Source
Click on the Sync toolbar.
OR
Choose Options-Project, and click the Clock tab.
Check the Audio box.
Click OK.
Best
John