Does subscribing to a shared internet service make me a partner in sin if one of the subscribers uses the internet to watch songs, movies, or similar things?

If you know or strongly believe that your neighbors will use the internet for unlawful purposes, and you cannot prevent them from accessing corrupt websites, then it is not permissible for you to involve them in it, because that would be considered assisting in sin, as Allah says: "And do not cooperate in sin and aggression." In this case, you would be a partner with them in sin.

However, if you know nothing about how they will use it, and there are no indications that they will use it for unlawful purposes, then you are excused. But if you later learn that they are using it for what is impermissible, then you must advise them, acting upon the words of the Prophet, peace and blessings be upon him: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith." And his saying: "Indeed, if people see a wrongdoer and do not restrain him, Allah is about to encompass them all with a punishment from Him."