The famous gemorah in Sukkah explains that in order that everyone should know when to answer Amen when the shliach tzibbur was davening in the shul of Alexandria flags would be raised since not everyone could hear.
However if not everyone could hear then how could they have been yotzee davening? They have to hear whatever brachos were being said and not just answer Amen?
Yehoshua, Yerushalayim, Eretz Yisrael
1. The Rishonim ask a contradiction between the account of Alexandria and between the Gemara in Berachos, and from the answers that they give to this question we can learn which Berachos one may be Yotzei even without hearing them and under what circumstances.
2. The Gemara in Berachos (47a) states that one may not answer an "Amen Yesomah" -- an "orphaned Amen." Rashi explains that this means that if one did not hear the Berachah itself but heard people answering Amen, one is not allowed to say Amen. Rashi questions this Halachah from the conduct in Alexandria. How could they answer Amen simply because they knew through the flags that a Berachah had been said? Rashi answers that in Alexandria, since they were aware that they were answering Amen, and they also knew on which Berachah they were answering Amen, it was not considered an "Amen Yesomah" even though they did not hear the Berachah itself.
3. Tosfos in Sukah (52a, DH v'Keivan) gives a different solution to the contradiction between the two Gemaras. He writes that the prohibition against an "Amen Yesomah" applies only to a Berachah with which one wants to be Yotzei. In contrast, the Gemara about Alexandria is referring to Berachos which the people were not obligated to hear and were not Yotzei with. An example of these are the Berachos said by the people who are called up to reading from the Torah. These Berachos are not an obligation on the congregation according to Tosfos, but only on the people who are called to the Torah. Tosfos writes that waving the flags did not help for Tefilah or for any Berachos which the Chazan recites in order to be Motzi the congregation through their listening.
4. Hence, according to Tosfos, it is clear that in Alexandria they were not Yotzei the Berachos that they did not hear. However, the Beis Yosef (Orach Chaim 124, end of DH Aval) writes that according to Rashi not only is one allowed to answer Amen if he knows what Berachah has been said, but he can even be Yotzei the blessing if it is one which he has an obligation to say, merely because he knows what Berachah is being said even though he does not actually hear it. The Beis Yosef adds that the Talmidei Rabeinu Yonah (Berachos, beginning of 35a in the pages of the Rif) writes explicitly that according to Rashi's answer one may be Yotzei a Berachah if he knows what Berachah the Chazan is saying. Knowing what Berachah the Chazan is saying is equivalent to hearing it. (The Talmidei Rabeinu Yonah write that since they said the Berachos in order, and the Chazan waved the flags, everyone understood which Berachah the Chazan was saying.)
5. The Shulchan Aruch (OC 124:8) rules like Tosfos, that if one is obligated in a Berachah but cannot hear it, he may not answer Amen to it even if he knows what Berachah the Chazan is saying.
6. To summarise, according to Tosfos, the people in Alexandria could not be Yotzei merely because of the flags, and the latter only enabled them to answer Amen on Berachos that they were not obligated to say. According to the Beis Yosef's understanding of Rashi, and according to the Talmidei Rabeinu Yonah, since they knew which Berachah the Chazan was saying, they could be Yotzei this Berachah even though they did not hear it.
Kol Tuv,
Dovid Bloom