Is listening to music that is forbidden by the consensus of the four schools of thought considered grounds for rejecting testimony, and does committing an act that compromises one's integrity necessitate sin?

Listening to singing accompanied by musical instruments is forbidden according to the unanimous agreement of the four Madhhabs, and this is evidenced by the rejection of the testimony of one who persists in listening to it. As for singing without an instrument, listening to it is considered makrooh (disliked) by the four Madhhabs, unless there is a fear of fitna (temptation/strife) from listening to it, in which case it becomes definitively haram (forbidden).

Among the proofs for the prohibition of listening to singing with instruments:

Ibn Qudamah (Hanbali): He mentioned that forbidden musical instruments (malahi) include strings, flutes, wind instruments, the oud, the tanbur, the ma'zifah (harp/lyre), and the rabab. He stated that the testimony of one who persists in listening to them is rejected. He cited a prophetic Hadith that mentions the open display of musical instruments and amusements as a cause for the descent of calamity. Asna al-Matalib (Shafi'i): It explicitly states that singing accompanied by an intoxicating instrument, such as the tanbur, oud, and all other musical instruments, strings, the mizmar (reed pipe), and the yara' (flute), is forbidden to use and to listen to. Radd al-Muhtar (Hanafi): It states that listening to the beating of the duff (tambourine), mizmar, and other such things is forbidden. Sharh al-Dardir (Maliki): It states that listening to singing with an instrument is forbidden.

As for singing without an instrument, some sources have mentioned that it is makrooh due to the amusement it contains. This is supported by the verse: "And among the people is he who buys lahw al-hadith (idle talk/amusement of speech)..." and Ibn Mas'ud's interpretation of it as singing. An exception to the prohibition was made for the singing of the two young girls on the day of Eid in the house of the Prophet, peace be upon him, because there was no instrument involved and it was on a day of celebration.

It should be noted that singing without an instrument by a non-mahram woman is even more disliked, and if there is a fear of fitna from it, then it becomes definitively haram.