In Sonar, go to Options - Audio - Driver Profiles. Make sure "Show Profile For:" has your Aardvark card selected. Change "Stream > 16 bit data as:" to "32 bit PCM, left-justified."

In Sonar, make sure "Always use MME..." is checked in Options - Audio - Advanced.

In Sonar, go to Options - Audio - Drivers and make sure all inputs of only one soundcard are selected. Exit & Relaunch the software.

Disable system sounds & Windows from using your audio card. Check our PC optimization guides by typing PC OPTIMIZATION in Sweetcare's Knowledgebase search for steps to do this.

Go into your Aardvark manager control panel and make sure that the sample rate matches what Sonar's sample rate.

Physically remove any Soundblaster cards you may have in your system. Some of these are known to conflict with Pro-audio cards.

Delete your AUD.ini from your Sonar directory (C:\Program Files\Cakewalk\Sonar1\ or C:\Program Files\Cakewalk\Sonar2\) folders.