Is it permissible to leave work at 2 p.m. instead of 3 p.m. to avoid music, knowing that the salary will not be reduced? And is this considered theft and injustice, or is there no obedience to a creature in disobedience to the Creator?

If your employment contract requires you to stay until 3 o'clock, you are not permitted to leave before that time without the administration's permission, due to the generality of Allah Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions."

As for music, if its presence in the workplace is obligatory and you cannot avoid it, there is no sin upon you as long as you do not intend to listen to it and you disapprove of it in your heart. This is sufficient, because there is a difference between intentional listening and unintentional hearing.