The reason for using a dedicated audio interface, rather than the on-board sound, is that there are massive amounts of data being converted back and forth between digital/analog, and the audio interfaces are explicitly designed to do all of that work with specialized circuitry, which means that the computer's CPU does not have to be burdened with all of that.
With NO audio interface present, all of that data conversion has to be handled by the CPU, which has a zillion other things to do, too. By overloading the CPU, it falls behind in it ability to process all of this data - which keeps getting thrown at it. The result is choppy playback, with lots of noise, crackles, pops, delays, and dropouts.
Windows can handle playing the standard Windows sounds and simple audio song files, or play YouTube videos, because those do not contain anywhere near the amount of data that you have in a Sonar project.
SOOOO - depending on the features you want, a decent sounding audio interface will start at around $125-$150, and go on up from there, Mine ran $500, but it is because of the extra features on it.
The ones that run in the $150 range will usually have a couple of inputs (usually a combo input, that can take in either a 1.4" instrument cable or an XLR cable), and a couple of outputs. Some have XLR outputs, some have 1/4" outputs, and some have both.
Look for something that has a couple of mic pre-amps and phantom power, which is used to power certain microphones.
Also, make SURE the interface has drivers for Windows 7, and should have them also for Windows 8 AND Windows 8.1, because some interface makers allow sometimes years to pass before building drivers for newer OS versions, and this can be a problem. The Windows 8.1 drivers may not be out yet, but certainly Windows 8 drivers should be available.
Most USB connected audio interfaces use USB 2 ports, so make sure you have one available on your computer. Some computers have only USB 3 ports, and these sometimes do not play well with audio interfaces that were expecting a USB 2 port. My Presonus AudioBox 1818 VSL freaks out if it gets plugged into a USB 3 port, for example.
When you DO get an interface, I would suggest the following starting point for settings:
For the interface settings: Sample Rate of 44.1 k or 48 k, and an ASIO Buffer Size of 128
For Sonar: Driver Mode of ASIO, Sample Rate that matches setting of audio interface, Record Bit-Depth of 24 Bits. Shoot for a Total Roundtrip Latency reported by Sonar, of around 10 ms or just a little less.
Bob Bone