You would probably do better to go to the real band site. But here are some questions.
The A-500 is the keyboard controller - what yamaha interface? Audio/Midi or both? The A-500 connects via Midi - so it must be plugged into this type of connector. From there the A-500 sends midi control messages, the basic ones being which note on (and off), velocity (how hard you play the notes to replicate piano soft/full effect) as well as many other kind of control messages.
The soundcard (what kind?) converts the midi notes to audio and streams any audio from your computer software (such as Real Band, which I have no knowledge of).
Answer how all these elements are hooked up and if you are trying to play your interface w/ the K500. Usually these built-in cards have a cheap General Midi synth which is used to play back game music, which is what I think you are trying to use (to play your own music, not game music, of course). Midi is just info, and could be assigned to your interface or a software synth in Real Band if it has such a critter. To many unknowns to give a more specific answer.
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