Is listening to backbiting, sometimes participating in it by speaking, or even remaining silent about it, considered as backbiting that necessitates repentance and prayer?

Abstaining from Allah's prohibitions is obligatory. The impermissibility of listening to backbiting and the method of enjoining one's parents have been previously explained in Fatwa no. 140554 and its related rulings. If you rebuke the one who backbites, then, Allah willing, there is no sin upon you, and you are not burdened beyond your capacity. We have clarified what is obligatory for someone present in a gathering of backbiting in Fatwa no. 131737. As for the prayer of repentance, it is prescribed after every sin, as we explained in Fatwa no. 59496, and it is not called the prayer of backbiting.