This の at the end of the sentence also appears as ~のだ / ~んだ, and serves the same purpose of asking for or providing an explanation:
よく聞くの? is casual for よく聞くのですか which can contract (without losing politeness) to よく聞くんですか
よく聞くの can be a casual statement as an answer. よく聞くのだ and よく聞くんだ are also casual (because they end in the plain form) but feels maybe a bit more masculine or forceful. よく聞くのです and よく聞くんです are the more formal ways to say this. I think ~の is a tiny bit more formal than ~ん since the latter is a phonetic contraction so a little more colloquial.
Honestly, I use this pattern a lot when I speak, because it makes it easy to speak in the plain form and not worry too much about conjugation, but also make it more polite if necessary by just throwing です on the end of the sentence