[On the immorality (fisq) of the singer and the listener to singing]

Answer (Allah is the One who grants success): Know, dear brother, that if you asked that scholar about the ruling on approaching women during menstruation, he would answer you with the same type of answer and would hesitate—despite its prohibition being by the explicit text of the Qur’an. The reason is that the scholars of the Ummah agreed that the perpetrator of a major sin is an immoral sinner, and they decisively judged him so; and that the perpetrator of a minor sin is not an immoral sinner.
But they differed in determining which sins are “major” and which are “minor.” It was said… and it was said… and it was said: “Whatever Allah threatened with the Fire or described as great is a major sin; and what is besides that remains open to being major or minor.” So the one who answered you may have answered on the basis of that view. And this answer does not mean that the prohibition of singing is doubtful or questionable; rather, the matter to him is as we described.
And it is obligatory to forbid singing and every act of disobedience—even if one does not decisively judge the doer immoral—such as singing, and approaching women in the anus, and approaching them during menstruation, and marrying a woman in ʿiddah, and entering homes without permission, and lowering the gaze from a non-maḥram woman, and winking, and kissing, and the like.
And this does not mean making them easy or treating them lightly. No act of disobedience should be treated lightly at all; rather, it should be forbidden with the strongest forbiddance.
If someone deliberately commits something from the minor sins and says: “There is no sin upon me in it,” then he is a disbeliever—on the condition that it has become clear to him that it is among what Allah—Exalted is He—and His Messenger (May Allah bless him and his family and grant them peace) forbade.
Now, what the author of al-Asās preferred is that “every deliberate sin is major,” based on the general threat against every one who disobeys Allah—Exalted is He—of abiding eternally in the Fire. So a believer should build upon this for himself, as a matter of caution; and then deal with people according to the first view—so he neither declares disbelief nor immorality—also as a matter of caution.
And know that a sin being “minor” does not affect the obligation of condemning wrongdoing. Rather, condemning wrongdoing is obligatory absolutely—by speech and action—according to the order (of levels), and Allah is the One who grants success.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3