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The Wiki page on Namco 163 audio lists some very approximate relative volumes for several games. A different page lists the mixing resistors used in the various games. I know that APU<->expansion audio balance is a hopeless affair, since it differs between Famicom revisions. But surely it would be possible to calculate the Namco 163 sound volume at least for the 10kΩ/15kΩ/22kΩ mixing resistors relative to 4.7kΩ mixing resistor configuration with precision. Unfortunately, I cannot. Can someone else?

The topology on the 163-using boards looks like this:
(edit: added location of Famicom expansion connector sound)

It should be the case that any given size of resistor should make the NES audio quieter proportionate to its conductance (1/resistance).

Unfortunately, the resistor values we've observed don't correlate with the reported loudnesses. So either the reported resistors are incorrect, or something even weirder is happening.

I thought that one could identify the resistor values from looking at high-resolution images, but no. The people on that archived nifty forum page linked to in the wiki seemed to just have held an Ohmmeter to the cartridges' audio pins ([MEMO 45-46の抵抗をテスターで計った結果] ).

One could, but the backs in NesCartDB don't match 1:1. The resistor is literally the only differnce between the different backs.