Firstly: 

Thikr in general, if it involves raising the voice and reciting in unison, is a bid‘ah (innovation) that has been introduced into the religion and was not the practice of the righteous early generations. 

Ash-Shatibi (may Allah have mercy on him) said: 

If there is a text that encourages people to recite particular phrases of thikr, and some people commit themselves to gathering to recite thikr in unison, or at a certain time that is singled out from all other times, then we should note that although these phrases were referred to in a shar‘i (religious) text by way of encouraging people to say them, there is nothing in that to indicate this particular way and time of doing it. Rather in that text there may be something to indicate the contrary, because adhering to something that is not binding in Islam may give the impression that this is part of the religion, especially if it is done by leading figures who are regarded as setting examples in places where people gather, such as the mosque. 

Therefore, the early generation did not commit to doing such things and it would have been more appropriate for them to have done these actions if they were prescribed in Islam. But thikr in general is something that is Islamically prescribed in many situations, unlike other acts of worship, and this du‘a (supplication) is thikr or remembrance of Allah, yet the salaf (pious predecessors) did not adhere to a particular manner of doing it or limit it to specific times, such that this thikr is connected to particular times, unless there is evidence for a particular time, such as the morning or evening. And they did not say it out loud unless the shar‘i text indicated that it should be said out loud, such as the thikr on the two ‘Eids (festivals) and the like. With regard to thikrs other than that, they persisted in hiding it and said it privately (not out loud). Hence when the Sahabah (Companions) raised their voices in du‘a, the Prophet (peace and blessings of Allah be upon him) said to them: “Be easy with yourselves (and lower your voices), for you are not calling on One Who is deaf or absent,” and so on. And they did not recite out loud or in unison. 

Everyone who goes against this principle has gone against the fact that the text is general in meaning and does not restrict it to a particular time, because the one who does that has restricted it on the basis of his own opinion and has gone against those who had better knowledge of Islam than him, namely the righteous early generations (the Salaf – may Allah be pleased with them). End quote. Al-I‘tisam. 

Shaykh al-Islam (may Allah have mercy on him) was asked about a man who established a waqf (endowment) and in some of his conditions he stipulated that they should recite whatever they are able to (of the Quran) and recite Subhan-Allah (Glory be to Allah), La ilaha ill-Allah (there is no God but Allah) and Allahu akbar (Allah is most great), and send blessings upon the Prophet (peace and blessings of Allah be upon him) after Fajr (early morning) until sunrise. Is it better to do this aloud or quietly? 

He replied: 

Praise be to Allah. Rather it is better to recite thikr and du‘a (supplication) – such as sending blessings upon the Prophet (peace and blessings of Allah be upon him) – quietly, unless there is a reason to do so otherwise. At this particular time it is especially preferable (to recite it quietly), because Allah says (interpretation of the meaning):

“And remember your Lord by your tongue and within yourself, humbly and with fear without loudness in words in the mornings.” [7:205]

And in as-Saheeh it is narrated from the Prophet (peace and blessings of Allah be upon him) that when he saw the Sahabah (may Allah be pleased with them) raising their voices in thikr, he said: “O people, be easy with yourselves (and lower your voices), for you are not calling on One Who is deaf or absent; rather you are calling upon One Who is All-Hearing and Close. The One upon Whom you are calling is closer to one of you than the neck of his mount.”

End quote from al-Fatawa al-Kubra. 

The scholars of the Standing Committee said: 

Thikr in unison is an innovation (bid‘ah) because it is something that has been introduced into the religion. The Prophet (peace and blessings of Allah be upon him) said: “Whoever introduces something into this matter of ours that is not part of it will have it rejected.” And he (peace and blessings of Allah be upon him) said: “Every newly invented matter is an innovation, and every innovation is going astray.” What is prescribed is to remember Allah, may He be exalted, without reciting in unison.

End quote. Fatawa al-Lajnah ad-Daimah. 

Secondly: 

It is more makrooh (disliked) to raise the voice in reciting thikr and du‘a, and to recite in unison, if this might annoy other people who are praying or offering du‘a or remembering Allah (thikr). 

It was narrated from ‘Abdullah ibn ‘Umar (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) observed i‘tikaf (seclusion in the mosque) and addressed the people, saying: “When one of you stands to pray, he is conversing with his Lord, so let each of you think of what he is saying to his Lord, and no one among you should raise his voice over anyone else when reciting in the prayer.” [Ahmad]

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: No one should raise his voice in recitation in such a way that he annoys others, such as other worshippers.

End quote. Majmoo‘ al-Fatawa. 

Thirdly: 

With regard to the report narrated by Muslim from Mu‘awiyah ibn Abi Sufyan (may Allah be pleased with him), according to which the Messenger of Allah (peace and blessings of Allah be upon him) came out to a circle of his Companions and said: “Why are you sitting here?” They said: We are sitting to remember Allah and praise Him for having guided us to Islam and blessed us with it. He said: “By Allah, are you only sitting for that purpose?” They said: By Allah, we are only sitting for that purpose. He said: “I did not ask you to swear because I am accusing you, but Jibreel came to me and told me that Allah was boasting of you to the angels.” 

And the report also narrated by Muslim from Abu Hurayrah (may Allah be pleased with him) who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “No people gather in one of the houses of Allah, reciting the Book of Allah and studying it together, but tranquility will descend upon them, mercy will overshadow them, the angels will surround them and Allah will mention them to those who are with Him.” 

–     There is nothing in these reports to suggest that thikr should be recited in unison. 

Al-Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

The correct view concerning this matter is that the two hadeeths (reports) refer to those who study the Book of Allah together and recite it. Similarly, with regard to people who are remembering Allah, it is general in meaning and should be understood in the light of other, specific reports that describe the way in which thikr was done at the time of the Prophet (peace and blessings of Allah be upon him) and his Companions. It was not known among them that they would remember Allah, may He be exalted, by reciting thikr in unison or that they read or recited the Quran in unison. The phrase “studying it together” indicates that this studying together is done one after another. Either one reads, and when he has finished the next one reads the same text, and so on; or each one of them reads one part, and the next one reads from where the first one stopped. This is the apparent meaning of the hadeeth. With regard to the other hadeeth in which it says that they remembered Allah, we say the same thing: it is general in meaning and should be understood in the light of the texts that speak of the specific way in which thikr was done at the time of the Prophet (peace and blessings of Allah be upon him) and his Companions. It was not known among them that they would get together and recite thikr in unison. 

End quote. Fatawa Noor ‘ala ad-Darb by Ibn ‘Uthaymeen. 

For more information please see the answer to question no. 10491 

And Allah knows best.