RunnerPack asked me about the specs on the audio levels (passed-through) on the cartridge. I didn’t remember ever checking that before, so I took a look at it tonight.
A little background… pins 55 and 56 on the cart connector are the audio source (right and left respectively) coming from the VSU (each through a 4.7k resistor). Pins 57 and 58 are the outputs, which go directly to connector P5 on the motherboard… which goes to the amp board.
The signal is DC coupled, with 0 scale seeming to be ~550mV, and typical audio values being approximately between 550mV and 750mV (this is before the amplifier, so the volume knob has no effect on this). I tested this with Mario Clash, not an actual test application, so I don’t know exactly what the full scale is… but this should give you an idea of approximate levels if you’re trying to mix your own signal into it.
I haven’t looked at the amp board, but its input is high impedance, and almost certainly AC coupled… so you can probably get away with a pretty sloppy mixing circuit.