Which son of Lamech was "the father of all who play the harp"?

Answer: Jubal

Jabal fathered tent-dwelling herdsmen, his brother Jubal fathered all who play the harp, and Tubal-cain forged bronze and iron.

“Adah gave birth to Jabal; he was the father of those who dwell in tents and raise livestock. And his brother’s name was Jubal; he was the father of all who play the harp and flute. And Zillah gave birth to Tubal-cain, a forger of every implement of bronze and iron. And the sister of Tubal-cain was Naamah.”— Genesis 4:20-22 (BSB)