misty fulcrum
Basically, just a block (maybe powered by rotation like the tracker bar) that relays note signals of one instrument to another. The functionality is as follows: The block has constant signal 2 filter inputs, one for the first redstone channel (red or blue) and another for another channel (same color), in addition to a right-click-and-drag modifier which determines the constant signal color (red or blue, frequency #1 or #2). The block will then work by relaying the signals from one set of channels (for instance, in the format (red/#1, blue/#2): Steam whistle + Red stained glass could relay to Vox humana + Red stained glass, and vice versa). It would then work the same, but reversed for the blue channels, if the 3rd input was reversed.