What is the position of the official in charge of social media accounts at the International Students Union who receives pictures, videos, and birthday greetings accompanied by songs, and does he bear the burden of sin for everyone who sees or hears them?

As long as you are responsible for accounts in which you publish clips and images that are impermissible to see or hear, it is not permissible for you to publish them. Repent to Allah Almighty for what you have previously published, and strive to delete it as much as you can. For what you are unable to delete, "Allah does not charge a soul except [with that within] its capacity."