First of all you need to establish that your soundcard is sending midi data to your Roland. Check in your windows control panel (under multimedia) that the MIDI playback goes to the midi output and not the internal synthesiser.
Now make sure you can hear your keyboard through your mixer. Just play the keyboard and check the audio is working.
Ok, if both of these are done, try playing a midi file on the pc using media player. It should play the keyboard. To combine the wave playback of the soundcard and midi output to the roland, you need to assign the midi outs of different tracks appropriately. Perhaps a cakewalk expert can help you out here.
Simon G.K. Williams
Creative Music & Multimediahttp://www.svpworld.com