i'm a newbie - embarrassed to ask, but i need help.

OK, I think this is what you are saying.  You are trying to play back different voices on the roland keyboard which is generating your sound (as well as being the controller).  SONAR and keyboard have to work together so they each know what the other is doing.  So your keyboard must be multitimbral - have the ability to play more than one voice, or kind of sound, at once.  There is usually a soft switch to enable multitimbral operation of a synth.  So the roland must physically be able to do this, as well as set to the multitimbral operation mode.  Then it has to be set up to recieve on different midi channels, so that the piano sound plays on channel one, while the guitar plays on channel 2, and so on and so forth.  Midi tracks aren't the same as midi channels - for your situation you want each track to play back on a different channel, which the roland is set up to recieve.  I don't know nothing about the keyboard and it is late, here, too late to look it up.  But I suspect your different lines of music for different instruments are on separate midi tracks but they are all playing on the same midi channel.  And that you physically changed the voices on the roland when you played them.  But SONAR and the Roland are dumb - you have to tell them to play the different instruments on differnet midi channels.  That is midi set, which is pretty simple in SONAR.  Read the help file about midi set up and midi channels.

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Looking at the manual, it appears as though there is a multitimbral General MIDI mode that will allow it to play multiple sounds, but as with most instruments, the manual is not very clear. You need to put it into a General MIDI mode and it will play 16 different sounds (1 per MIDI channel). Read the manual carefully and experiment. You also might find some more info on the web so Google rd300sx and see what pops up.