If, in the middle of his recitation, he hears a berachah, he should answer amen. However, if doing so confuses him in his recitation, or his time is short to finish the recitation, etc., he is exempt from answering amen.
Sources and Explanations
Rabbi Shmuel HaLevi Wosner, in Responsa Shevet HaLevi (vol. 9, siman 43), was asked about a person standing in his prayer at a point where he is permitted to answer amen, for example before “Hodu,” who hears other worshippers reciting Birkot HaShachar (the morning blessings): is he obligated to answer amen? He wrote that if it is difficult for him because of the confusion or for other reasons, he is not obligated.
To explain the matter, one should know that our masters the Beit Yosef and the Bach (Orach Chaim, siman 215) disagreed whether answering amen is an obligation or optional. In the view of the Beit Yosef it is optional, and according to the Bach it is an obligation. The Bach proved this from the words of the Rambam, who wrote that it is an obligation. The Shevet HaLevi wrote that the wording and style of the Tur indicate more like the Beit Yosef.
Therefore, although according to halacha one should answer amen after the one reciting the berachah, nevertheless, where it disturbs him in his prayer or confuses him, one may be lenient. This ruling holds according to all opinions, for we learned in Berachot (53a) that if people were sitting in the beit midrash and a light was brought before them, Beit Shammai say each one recites the berachah for himself, and Beit Hillel say one recites it for all, because of “In the multitude of people is the King’s glory.” Since they disagreed only regarding the principle of “the multitude of people,” this implies that all would agree that where it would cause bitul beit hamidrash (interruption of study in the beit midrash), one should not interrupt. The same can be said regarding tefillah.
From the sefer “Shaf Veyativ – Laws of Saying Tehillim” by Rabbi Shay Tahan, Chapter 7: What Is Forbidden and Permitted When Saying Tehillim, Halacha 11 (p. 126 in the print edition). Translated from the Hebrew original.
