You shouldn't have to download anything else, Brandon. I use ASIO4ALL on all my little internet boxes that have Realtek soundcard on them without any issues.
All you have to do is install asio4all, and after you do that, opening Sonar you'll need to go to your audio preferences and click on the playback and recording options. You probably have WDM selected...choose ASIO. Once that's done, you should see a little green icon appear in your task bar. That is the asio4all control panel icon which you will need to set up by clicking on it. Once you set that up, cose down Sonar, reopen it and then make all the correct settings inside your preferences/audio options.
For drivers, all you need is line is for your input...don't use mic in. And for output, ASIO 1/2. That should get you working. Here are some screen shots of how my ASIO4ALL is set-up. I run this exact configuration on all my Realtek PC's.
In this shot you will see my ASIO4ALL control panel configuration (seen by clicking on the green icon on your desktop that will appear when Sonar is running) as well as some driver settings.
Closer look:
https://dl.dropbox.com/u/4909348/ASIO4ALL%201.JPG In this shot, some more settings.
Closer look:
https://dl.dropbox.com/u/4909348/Settings.JPG Hope this helps you, good luck.
-Danny