Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) recited «وَلَاالضَّالِّينَ», he would say Ameen, and when saying it, he would raise his voice.
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1281
That is, the Messenger of Allah (sallallahu alayhi wa sallam) used to say "Ameen" aloud.
This is established by authentic narrations, and the Ahl al-Hadith act upon this. In the sacred cities of the Haramain as well, this is the practice, and this alone is the Sunnah of the Messenger (sallallahu alayhi wa sallam). It is in exact accordance with the statement of the Prophet (sallallahu alayhi wa sallam): «صَلُّوْاكَمَارَأَيْتُمُوْنِيْأُصَلِّيْ.» "Pray as you have seen me praying."
May Allah grant everyone the ability to act in accordance with the Sunnah of the Messenger (sallallahu alayhi wa sallam), Ameen.