Having been in a few different ancient Roman theatres I'll speak only for how they'd address crowds in a theater context. The amphitheaters are incredibly well made for acoustics. You can stand in the center and speak in a normal voice and be heard throughout the theatre.
They also used to have relayers that would know what was going to be said and repeat the information from hidden rooms underneath the seats.
From the last time this was asked. https://www.reddit.com/r/AskHistorians/comments/1tfwsn/before_the_age_of_electronic_voice_amplification/ce7j3wc
The answers seem to address how speaking to the masses worked in theatres, but how did it work on the battlefield?
FYI, you'll find a section in the FAQ on this topic