How exactly would I need to modify the cart? And since I want it to work with any NES, so I don't want to mod the NES.

Would it be possible for me to modify the cart so that the MMC5 audio runs to the expansion port (via one of the the EXP pins of the cart connector) and then make an expansion port device and plug it into the expansion port that would take the data from the NES Cart connector (the audio data) and then run that to the audio out pin of the expansion port (thus no modifying of the NES needed, just plug a device into the expansion port that runs the data to audio out pin)?