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.