Mizmar means flute but here it stands for tune and sweet voice. In the phrase Al-Dawud (PBUH), translated here 'the family of Dawud,' the word Al is superfluous and what is in fact meant is Prophet Dawud. In any case, sweet voice is a gift of Allah Who grants it to whom He likes. Lucky indeed are those who are granted this gift and they go about using it to invite people to the religion of Allah.