Is a listener of songs who does not deem them permissible exposing himself to being transformed into a monkey or a pig, and in what situation would that occur?

Singing has different types, and each type has its own ruling. As for musical instruments and wind instruments, scholars have reached a consensus on the prohibition of listening to them, with the exception of the duff. Both quiet and loud musical instruments are prohibited. However, if singing is without a musical instrument, and its words have good meanings, and it is not from a woman in the presence of men, then there is no harm in it.

The punishment of transformation (maskh) is affirmed in the Sunnah. Among the narrations is the hadith from Imran ibn Husayn that the Messenger of Allah, may Allah's prayers and peace be upon him, said: "Among this Ummah there will be sinking into the earth, transformation (maskh), and pelting [with stones]." A man from the Muslims asked: "O Messenger of Allah, and when will that be?" He replied: "When singing-girls and musical instruments appear, and intoxicants are drunk."

Those who listen to musical instruments and love them will be transformed into apes and pigs if they deem them lawful through corrupt interpretations. It is likely that their deeming musical instruments lawful stems from their belief that musical instruments are merely hearing a sound that contains pleasure, and that this is not prohibited, like the melodies of birds.