Surah Al-Hujuraat (49) — Ayah 2

O you who believe! Do not raise your voices above the voice of the Prophet, nor speak loudly to him as you do to one another, lest your deeds [3] become worthless while you are unaware.

[2] That is, when you are sitting in the gathering of the Prophet ﷺ, you must observe his respect and reverence. The reason for the revelation of this verse can be seen in the following hadith: Ibn Abi Mulaykah says that due to raising their voices before the Messenger of Allah ﷺ, two of the best men were about to be ruined, namely Sayyiduna Abu Bakr Siddiq ؓ and Sayyiduna Umar ؓ, when a delegation of Banu Tamim (in 9 AH) came to you (and requested you to appoint a leader for them). One of them suggested the leadership of Aqra' bin Habis, who was from Banu Mujashi' (a branch of Banu Tamim), and the other suggested someone else (Qaqa' bin Ma'bad). Nafi' bin 'Umar says that I do not remember his name. Upon this, Sayyiduna Abu Bakr Siddiq ؓ said to Sayyiduna Umar ؓ: "You only want to oppose me." Sayyiduna Umar ؓ replied: "I do not want to oppose you" (rather, it is a matter of expediency). In this matter, both of their voices were raised, so Allah Almighty revealed this verse. Sayyiduna Abdullah bin Zubair says that since this verse was revealed, Sayyiduna Umar ؓ would speak so softly that the Prophet ﷺ would have to ask him what he said. But he did not narrate this about his grandfather (Sayyiduna Abu Bakr ؓ). [بخاري۔ كتاب التفسير]
Although this etiquette was taught for the gathering of the Prophet ﷺ and its addressees were the noble Companions or those who were present in his time, and this etiquette was taught so that people would not consider him an ordinary man, but rather understand that they are sitting in the gathering of the Messenger of Allah, nevertheless, this command also applies to occasions where the Prophet ﷺ is being mentioned, or any of his commands are being conveyed, or his ahadith are being narrated.

[3]
The Voice Should Be Comparatively Lower:

Its second aspect is that even if you have to speak to the Prophet, your voice should not be louder than his. Also, in Masjid Nabawi, no one should speak in a voice louder than the normal tone. The punishment for this disrespect could be that your good deeds are rendered void. The effect this verse had on the noble Companions is evident from the following hadith:
Sayyiduna Anas bin Malik ؓ says that the Messenger of Allah ﷺ did not see Thabit bin Qais (bin Shammas) in his company for several days. A man (Sa'd bin Mu'adh) said: "O Messenger of Allah! I will find out about his condition and inform you." So he went and saw Thabit ؓ at his home with his head bowed and asked: "What is the matter?" Thabit ؓ said: "It is bad for me; my voice used to be louder than the Prophet's, my deeds have been wasted, and I am among the people of Hell." Sa'd ؓ came to the Prophet ﷺ and informed him: "This is what he says." Musa bin Anas says: Then it happened that Sa'd bin Mu'adh went to Thabit bin Qais again with great glad tidings. The Prophet ﷺ himself sent Sa'd ؓ to Thabit and said: Tell him that: "You are not among the people of Hell, but among the people of Paradise." [بخاري۔ كتاب التفسير]
This Thabit bin Qais ؓ was the orator of the Ansar. When Musaylimah the Liar came to Medina to negotiate with the Prophet ﷺ on the principle of give and take, the Messenger of Allah ﷺ appointed Thabit bin Qais to speak with him. His voice was naturally heavy and loud. That is why he became afraid of this command. The Prophet ﷺ exempted him from this command because he did not raise his voice out of disrespect or lack of reverence, but rather his voice was naturally loud.