Is there a possible way to do this without modding the system? Pin 54 seems to be the pin that takes the audio info in the NES. And that pin 54 connects to the expansion port pin 9. Would it be possible to like plug something into the expansion port that ran pin 9 (MMC5 audio data) to pin 21 (audio out) so that the NES can play MMC5 audio without modding the NES in any way (just plugging something into the expansion port. thx ^_^

There are audio in and out pins on the expansion port, I suppose you could run the signal through an expansion line and connect it down there. But it doesn't seem any easier than modifying the system internally. The cart(s) and the system both need modified either way you do it.

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)?

Yeah, something like that should work. You might need to take the 'audio out' and mix it with the cart sound, then connect that to 'audio in'. I'm not sure, since I haven't done anything with the expansion port. I don't know where to get a connector for it, do you?

"I don't know where to get a connector for it, do you?"

No, but I was thinking about modifying the 72-pin cartridge connector by cutting it up to make a 48-pin connector and then run pin 9 through to pin 3 (audio in) and then connect it to the expansion port.

That or I'll have to make my own connector and run pin 9 (pin 54 from cart connector) to pin 3 (audio in).

"You might need to take the 'audio out' and mix it with the cart sound, then connect that to 'audio in'"

Actually anything running through audio in I think mixes with 2A03 produced audio anyway (as said in Brad Taylor's NES hardware development guide).

Do you know exactly how I'd have to modiify the MMC5 cart?

I'm not sure about the exact modifications. You could ask drk421 in this thread, since he's done it.
http://nesdev.parodius.com/cgi-bin/wwwthreads/showpost.pl?Board=hardware&Number=692