Is the punishment of pouring molten lead into the ear of whoever listens to singing specific to those who regularly listen to it, or does it include those who listen to it rarely or persistently?

There is no authentic hadith about the punishment for someone who sits with a "Qaynah" (a singing-girl or female musician). The hadith narrated from Anas concerning this matter is false and fabricated. Simply listening to music is not originally considered a major sin, though it may become so depending on a person's circumstances.