1- Is it a sin to listen to a conversation that you’re not a part of where they are backbiting others, and if it is a sin, are you obliged to stop them from backbiting, or is it sufficient upon you to just walk away?
2- If I am part of that conversation, and they bring something up that leads to backbiting but I do not speak, is it a sin for me if I continue listening to that conversation? In this case am i obliged to stop them, or can I just be quiet?

1- Yes, it is not permissible for you to listen to backbiting and keeping silent, rather you must forbid that, and if you are not able, then you must walk away.
2- Same as in 1