Certain famicom cartridges have upgraded audio chips in the system. However to mix in the audio from these famicom carts the famicom cartridge connector has the system audio pass through two pins. Pin 45 of the famicom cartridge connector is mono system audio output and pin 46 is the mono system audio mixed with the cartridge upgraded audio. However the nes cartridge slot doesn't have the audio path connect to the cartridge pins. The solution is to build a famicom to nes adapter that has pins 45 and 46 of the famicom connector wired into unused pins on the nes cartridge connector (unused expansion port pins work great). Then you need to wire the nes mono system audio output into whichever expansion pin connects to the adapters pin 45. The expansion port pin that connects to the famicom adapters pin 46 will be your audio output that is mixed with the famicom cartridge. Unfortunately nes cartridges don't bridge these two connections so you'll have to wire up a switch that switches between outputting the regular system audio to your rca connector and the famicom cartridge circuit outputting to the rca connector.
Sorry but I'm too lazy to take pictures for this circuit.