Question: Some people say, "Hadrat Dawud used to perform dhikr in company with musical instruments. All today's Muslims, including those in Saudi Arabia and other Muslim countries, employ the use of loudspeakers in acts of worship. Is it possible that such a large number of people may err?" Does everybody's using it not show that there is consensus (ijma') on the permissibility of it?
ANSWER
Mizmar means musical instruments. The loudspeaker is a mizmar, too. Plenty of hadith-i sharifs inform us of that the Qur'an al-karim will be recited through instruments by which music is played, such as stereos and loudspeakers. Reciting or listening to the Qur'an al-karim through a loudspeaker means changing the act of worship. That is, it is bid'at and sinful. (Se'adet-i Ebediyye)

Everybody's employing the use of loudspeakers in acts of worship does not prove that using it is permissible. The one who follows the majority of people is in the wrong. The Qur'an al-karim says (what means), "If you follow the majority of people on earth, they will lead you astray from the way of Allah" (Surat al-An'am 116).

The consensus of today's people cannot be called ijma'. It is written in our Islamic books:

There has been no mujtahid mutlaq (full-fledged mujtahid) after the fourth century, and there has been no ijma’ (consensus) since then. Unanimity among the muqallids, the ignorant, or especially among the religion reformers cannot be called ijma’. (Faideli Bilgiler [Useful Information])

Just as taghanni is sometimes haram and sometimes permissible, so mizmar also means a lovely voice. Dawud ('alaihis-salam) had a beautiful voice. A person who has a beautiful voice is called the one with Dawudian voice. Dawud ('alaihis-salam) used to recite the Zabur with a beautiful voice. Claiming that he used to recite it in the company of musical instruments is grave slander. Once, Abu Musa al-Ash'ari, one of the Blessed Companions, was reading the Qur'an al-karim. The Messenger of Allah heard it and said, "His voice is exactly the same as the mizmars [beautiful voices] of the family of Dawud." (Majma-uz-Zawaid)

In the hadith-i sharifs below, the word mizmar is used in the meaning of all kinds of musical instruments and loudspeakers:

(Allah curses those who recite the Qur'an al-karim by mizmars.) [Musamara]

(Be hasty in doing good deeds before the Qur'an is recited by mizmars.) [Tabarani]

(When the Qur'an is recited by mizmars, die if it is possible.) [Tabarani]

Besides, in our religion there are many prohibited things that were permissible in former religions. For example, consuming alcohol was not prohibited in some of them. Saying "Playing musical instruments is permissible in Islam, too, because it was not prohibited in one of the former religions" is similar to saying "Consuming alcohol was permitted in the past, so how can it be prohibited now?" Some pertinent hadith-i sharifs are as follows:

(The bell is the mizmar of Satan.) [Muslim, Abu Dawud, Nasai]

(Allahu ta'ala said to Iblis, "Your muadhdhin is mizmars.) [Tabarani, Ibn Jarir]

(A time will come when those who deem fornication, alcohol, and mizmars to be lawful will appear.) [Bukhari]

(I have been sent to eradicate idols.) [Imam-i Ahmad, Abu Nu'aym, Ibn Najjar]

(It incurs the wrath of Allah to play a mizmar when one attains a blessing.) [Daylami]