Ok, good. Thanks, that's important.
I need to check the manual on the SD50, but you have given us something to work with.
Bounce is not an option with external hardware. That only works with softsynths.
You should be able to record the audio output in real time from the SD50(which you have acknowledged).
The kicker is going to be how the SD50 presents its synth outputs to Sonar.
I'll be back.

Well, in Roland style, the manual is somewhat vague.
It appears that your synth may have 4 virtual outputs (1L,1R,2L,2R). You would need to assign the instrument outputs in the SD50 to one of these.
in Sonar8.5, click Options>Audio>Drivers.
What do you see?

Best advice I can offer is to forget the Roland Sound card and try inserting the TTS-1. It will work much better for you and the sounds are actually based on Roland sounds anyhow so they will be the same if not better. The world of soft synths is much better than using a sound cards lame midi as a source. We left that behind in 1998?   
Go to VEIWS/ SYNTH RACK/  + soft synths  / TTS-1 and choose the "all synth audio output mono" in the dialog.  
 

Hay all.
Not sure about this, I did try it out, and it seemed to work for me...as I do not know how to yet bounce midi to audio yet in sonar.
 
I opened up a old midi file of xg type to play with vsti and get to work on setting it up with differing things like AD2 for drums.
 
I was able to select the AD2 output as a audio track input...and record it.
Did that for the bass, Viola, and Cellos....Worked ok for me so far.,....But I do have a very good audio setup with lots of ways to route things.
 
Just thought I would chime in...just in case all else fails.

I think what you're really trying to do isn't bouncing at all, but rather recording a sound module's outputs while simultaneously triggering it.  If the SD50's on-board sounds can be routed to its outputs, you could send those outputs to the inputs of audio tracks in SONAR and record those results while the MIDI tracks are sending their data to the SD50.   Assuming the SD50 can do this, of course. 

That's it in a nutshell, Steve.
Even the Op suggested he may be barking up the wrong tree in an earlier post.
It boils down to what outputs the SD-50 presents to Sonar.
As Johnny noted, it would be much easier to use VST instruments such as TTS-1.
The exciting thing here is that I think Paul is on the verge of discovering just exactly what he has his hands on.