Does anybody know what the C21 on a Double Dragon board does? The game plays fine but its missing the gameplay music. Does this cap have anything to do with the audio?
C21 is just a ceramic 104z smoothing cap between a 74ls163 and another 74 logic chip, the board will be fine without it. It just provides better tolerance for power fluctuations.
Missing music on a DD is usually due to the -5v being missing.