Title says it all. Can you send any mono audio through to the Famicom using the audio input pin on a cart?
Not on the input pin (pin 45) but you can send mono audio out from the audio output pin (pin 46) on the cartridge.
The APU sends amplified audio to the input pin (pin 45) and the cartridge needs to send it out on the output pin (46) to the final audio output which goes to the speakers/RF-modulator.
Most games just bridges pin 45 and 46 so that audio passes through directly to the speakers, but some games takes the audio input (pin 45) and mixes it with audio generated by a sound chip inside the cartridge and then sends it out from pin 46 to the speakers.
Cartridges that does mix in extra audio includes the FDS RAM Adapter, Akumajou Densetsu, Gimmick!, Just Breed, Lagrange Point and a bunch of Namco games.
A list of all known games which mixes in extra audio here (https://wiki.nesdev.com/w/index.php/List_of_games_with_expansion_audio).
Thanks! just got them mixed up. I had this idea for a Famicom MP3 player for a while, and it's great knowing that that idea can come to fruition.