I've seen there are adapters to convert Japanese Famicom games to work on the NES. You may have to remove the lockout chip, I can't remember.
Anyway, many Famicom games included extra sound hardware to enhance the in-game music. However, on the NES, this extra sound cannot be heard because the NES lacks the circuitry to route the extra audio from the game cartridge to the NES audio circuitry.
My question is, was there any 3rd party adapters that would not only route the Famicom game to the NES console, but also route the extra audio signals out to perhaps an external audio port on the adapter so that we could mix the two audio signals together?