halacha - Can you fulfill the obligation to pray if you can't hear the leader?

The gemorah in Sukkah daf 51b explains that, in the enormous shul in Alexandria, in order for everyone to know when to answer Amen when the shliach tzibbur (prayer leader, aka sha"tz) was davening, flags would be raised since not everyone could hear the sha"tz.

But if not everyone could hear, how could they have fulfilled davening? They have to hear whatever brachos were being said in order to be "yotzee" and not just answer Amen.

For reference on the halachos of being yotzee by the Shliach Tzibbur, see Shulchan Aruch Orach Chaim Siman 124 Sif 1 and Mishneh Brurah there (Sif-Katan 2).

In the gemara's time I have no doubt that this method of prayer (which is now practiced only in certain communities) was still widely being done.

Answer

After some investigation I think I've found what could be a reasonable answer in light of the kasha that one could have on the gemorah. How is it they were "yotzee" davening if they couldn't hear the shliach tzibbur?

Look into the Yerushalmi Sukkah Perek 4 Halacha 1 (in my print this comes out on daf 23a a little down from the top.) The version over there the case was that they were leining from the Torah and someone would then alert the tzibbur when to answer Amen on the brachos of Krias HaTorah (according to the Yerushalmi we weren't even talking about Shemoneh Esrei.) Now if one looks into the Karbon Ha'Aeeh you will see this pashut. He does add "v'ee nami" on the brachos of the shliach tzibbur. However I think he is perhaps forced to add this pashut in only because of the traditional understanding of the bavli. However I want to say that without the Yerushalmi you can't begin to understand what the case was exactly. Is there any proof there from the Bavli that it's speaking about Shemoneh Esrei davka? It just uses the word "brachos" However clear in the words of the Yerushalmi that it's talking about Bircas HaTorah.

From the Rosh in Brachos Perek 7 Siman 17 one could say not like this and that pashut in the gemorah was that it's speaking about Shemoneh Esrei. However he also says there that perhaps the gemorah was speaking about when they were already yotzee davening.

So between Yerushalmi and the Rosh there seems what to be in order to answer this kasha on the gemorah.