Without seeing how your mod was done, I can only guess. So a few things to look at. If you are using the amplified audio from the NESRGB board, then you need to route a resistor into that audio circuit from pin 51 of the toploader. If I recall correctly it should be 47k ohm. If you are not using the audio from the NESRGB board, then as long as you have the resistor from pin 51 connected to the audio you are feeding out of the system, you should be fine. In the famicom to nes cart adapter you shouldn't be adding any resistors, all you need is to connect pin 46 of the famicom side to pin 51 of the nes side. You may also need to connect pin 54 to pin 51 on the nes side for MMC5, but that depends on your setup and the cart adapter.
If you are using the NESRGB audio, Tim states that you really should build a different audio circuit to use with his amplified circuit. Here is a link to the circuit Tim says to use for the NESRGB board.
https://shmups.system11.org/viewtopi...15605#p1115605