Power/AV Boards - Why cutting trace to Pin 45?

[Krystman]

I've been replacing the RF Boards on a few Famicoms with new Power/AV boards. I've been looking at some common designs and thinking for building my own from scratch. Here is something I don't quite understand, maybe someone knows what is happening:

I've seen some advanced Power/AV boards do a little trick with the audio signal. Instead of piping the audio from the Famicom straight into the AV connector, they do this sketchy thing where you are supposed to CUT a trace on the Famicom PCB leading into pin 45. And then you are supposed to hook up Pin 46 to the board. And then there is often some other place in the front that needs to bee hooked up (AUX IN?).

Here is an example. This is used together with the Chinese Lava RGB mod
http://www.lava-fc.top/LavaRGBKitInstallOnFamicom.pdf

The NESRGB board does somethings similar
https://etim.net.au/nesrgb/installation-famicom/



I've also seen at least one other board do something similar.

This seems like a very complicated and destructive procedure. Why is this being done?

I understand that Pin 45 is the audio from the CPU and that is being sent to the cartridge. And then some cartridges have expansion audio which is then being mixed into that and sent out on Pin 46. So when you cut the trace, Pin 46 will only output the expansion audio, right?

So then the AV Board is doing the mixing itself? But why? Is the mixing done inside the cartridge somehow inferior?

Also what is the AUX_IN signal for the Lava RGB mod? It's so far in the front. Is this somehow perhaps related to the microphone on the controller?

[P]

Hmm that is indeed very strange and seems destructive. The mixing in the cartridge depends on the game and may have different output even on different games of the same developer.
If the AV board has its own mixing I guess it must have a potentiometer for manual adjustments for different games like you need for an expansion audio mod on a NES.