A while back I posted my quick expanded audio mod for my NES and mentioned I might try other levels of resistance to see how things sound.
Ultimately I decided to wire up a three way switch to toggle between 47K Ohms, 94K Ohms, and unmodded (basically so I can "remove" the mod easily if I want to).
Pretty happy with this so far and it requires no permanent modification to the shell. The wiring is easy to run through the bottom by the expansion port and the cover will clip back on perfectly around the wires. I bundled the wires outside the cover with some tubing and hot glued the switch to the edge of the NES shell.
I apparently didn't take a photo of the actual wiring at the board but the basic idea is this (from memory so bear with me!): Using my 3rd pic as a guide, the green wire is soldered directly to pin 3, the brown wire is soldered to pin 9 with a 47K Ohm resistor, and the grey wire is soldered to pin 3 with another 47K Ohm resistor. So that means the top position is "unmodded," middle is 47K Ohm resistance, and bottom is 94K Ohm resistance.
The above will make more sense if you're already familiar with the mod (all you're doing is applying resistance between pins 3 and 9 of the expansion connector). I tend to leave it set to 94K but your preference may differ!