Is it true that music and fornication began in the era of Seth, and that Satan was the one who introduced music and drums to the people of Cain, and is Seth a real figure mentioned in authentic hadiths?

No fixed text in revelation reports about Seth and what happened in his era. Most of the narrations related to that are taken from the People of the Book, including what Al-Tabari mentioned about the emergence of fornication and music between Adam and Noah, peace be upon them. It was mentioned on the authority of Ibn Abbas that immorality spread among Adam's offspring due to women's display of their adornments and their mingling. Since these narrations are Isra'iliyyat (from Jewish sources), they are neither to be believed nor disbelieved. Instead, one says, "We believe in Allah and what has been revealed to us," acting upon the guidance of the Prophet, peace and blessings be upon him.