Is it considered apostasy for a Muslim to jokingly say, "This song is beautiful, it's permissible for us to listen to it," while believing that songs are forbidden?

It is not permissible to jest by declaring what Allah has forbidden as lawful, especially when one is aware of its prohibition. This is based on Allah’s saying: "And do not say about what your tongues falsely describe, 'This is lawful and this is unlawful,' to invent a lie against Allah. Indeed, those who invent a lie against Allah will not succeed" (An-Nahl: 116). This is not considered an act of disbelief (kufr) through deeming something lawful, as deeming something lawful to the extent of disbelief is the belief that something forbidden is lawful. For example, if someone believes that cursing is permissible, or that prohibitions known to be forbidden are permissible, and that the ruling is known to be an essential part of the religion. However, the prohibition of music is not known as an essential part of the religion. Nevertheless, this might fall under mockery and belittling the rulings of Allah, which is a great danger, as in Allah’s saying: "Say, 'Is it Allah and His verses and His Messenger that you were mocking?' Make no excuse; you have disbelieved after your belief" (At-Tawbah: 65-66). Therefore, delving into the Book of Allah, His Messenger, and the attributes of Allah, and making them a subject of play and mockery, is an act of disbelief that expels one from the fold of Islam. A Muslim should beware of this, for a word might seem insignificant, but if it belittles the Sacred Law, it is disbelief.