12V MUX with single output
If I understand correctly you're still using each of the signals to drive the LEDs and need to combine them to turn on a single buzzer or light. You can do this with buffers (or inverters for active low signals, not shown). Simply pass each signal through a buffer and combine the outputs to drive the single load device. A discrete component view might look something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

You can use three CD4050Bs (HEX buffers). This will provide easy expansion for up to 18 inputs (I would put them all on the PCB and, for now, only populate the 14 that will be needed). If your signals are active low, use three CD4049UBs (HEX inverters) instead and hook up the LEDs as you've drawn them in your schematic.