You can make explicit that Alexander was showing the crowd that he wanted them to be quiet. AT: "gestured to the crowd to be quiet" (See: [[rc://en/ta/man/translate/figs-explicit]])
The shouting together of the people at the same time is spoken of as though they were speaking with one voice. AT: "in unison" or "together" (See: [[rc://en/ta/man/translate/figs-metaphor]])