Is it obligatory to denounce those who commit forbidden acts, such as listening to music, if the official fatwa-issuing body in the country permits it?

The criterion for enjoining good or forbidding evil, or refraining from it, is not the preference of the fatwa-issuing body in a given country. Rather, it is the existence of a legitimate difference of opinion among scholars on the issue, followed by the individual's own conviction.

In matters where there is a legitimate difference of opinion, one should not condemn the doer if he believes the act to be permissible. However, if he believes it to be forbidden, then it is obligatory to condemn him. If he holds no conviction regarding its permissibility or impermissibility, then it is recommended to advise and guide him.

When enjoining good or forbidding evil, one must take into account the circumstances and weigh the benefits against the harms.

To determine whether the difference of opinion regarding musical instruments is legitimate or not, one can refer to the mentioned fatwas.