The fuqaha’ differed concerning how the Ameen is to be said by the one who is praying behind an imam in Qunoot al-nawaazil.
The Hanafis are of the view that there is no Ameen in Qunoot al-nawaazil, because the Qunoot should be recited silently unless the imam recites Qunoot out loud.
The Shaafa’is and Hanbalis are of the view that the Ameen should be said out loud in Qunoot al-nawaazil.
According to the Maalikis, Qunoot al-nawaazil is not prescribed in the first place, and the Ameen belongs to the Qunoot; if the thing it belongs to is cancelled then it is cancelled too.
See: al-Mawsoo’ah al-Fiqhiyyah (1/116).
It says in Mughni al-Mughtaaj (1/362): Note: The one who is praying behind the imam should speak out loud in five places: four places of saying Ameen: he should say Ameen with the imam (i.e., after he recites al-Faatihah); in his du’aa’ when he recites the morning Qunoot; in Qunoot of Witr in the second half of Ramadaan; and in Qunoot an-Naazilah (Qunoot of calamity) in the five daily prayers; and if the imam makes a mistake in his recitation and a person praying behind him corrects him. End quote.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: If we say that Qunoot may be offered in the five daily prayers, if the prayer is one in which Qur’aan is recited out loud, then it is well known that the Qunoot should be recited out loud, and if the prayer is one in which Qur'aan is recited silently then Qunoot should also be recited out loud, as it is proven in the Sunnah that the Prophet (peace and blessings of Allaah be upon him) used to say Qunoot and the people would say Ameen behind him, and they could not have said Ameen unless he was saying Qunoot out lout. Based on this, it is Sunnah to recite Qunoot out loud even in a prayer in which Qur’aan is recited silently. End quote from al-Sharh al-Mumti’ (4/47).
Thus it is known that if the imam recites Qunoot out loud, it is prescribed for the person praying behind him to say the Ameen out loud, but he should not go to extremes in saying it out loud, in such a way that is contrary to proper focus (khushoo’) and tranquillity, as some people do, raising their voices as if they are on a demonstration and not at prayer.
It is not obligatory to say the Ameen out loud, rather it is mustahabb, just as the Ameen itself is not obligatory.
And Allaah knows best.