The output from the SD will only be as good as the audio you get from your audio device, because you have to route it through the audio device.
If the SD 50 is similar in architecture to the SD 20, which I have, its a midi generator and can, within its own editor (or via audio out sockets) generate audio sound. For SONAR you'd need a soft synth.
Within Sonar, once you've installed the drivers, you can choose the SD20/50 as the midi sound generator - in other words route a soft synth through it, and then on outpût it to your sound device. You'll need an input device (i use a PCR 800 USB) either a keyboard or PRV in Sonar, not by note. Its complicated to set up but once done it works ok.
BUT, at the end of the day, I find I really don't need the SD20 bit, as the Sonar soft synths can replace it if you use a midi keyboard .....
BTW, on the SD20 there is a Midi through (to use with a keyboard controller for example) and a stereo audio output (two cinch sockets) which are capable of taking audio to an audio device.
All this without actually doing any research on the SD 50 ......
Përsonally I think to invest in a midi keyboard/controller makes more sense than using my SD 20 in todays DAW world. I rarely light it up.
Cheers,
Jerry