I've heard that Muhammad (PBUH) gave speech before 120,000 Muslims at Ghadir at the final Hajj of him. So, for how many people was it possible to hear what he was saying? Even if one spoke loudly in front of 1000 people, they would have difficulty to fully understand the speech, let alone 120,000 people. So how was anybody hearing anything at all?
I don't know whether people where able to hear him or not, however that isn't impossible taking into consideration the possibilities below:
They may have used a communication method similar to that used by a historic military leader to address an army of 100,0000 or more, they must have used some method to assure that everyone received the instructions issued by a leader(will find what that method may be)
The position of the speaker and landscape can have an influence on of where the sound waves may travel and reach, (i.e. those standing at
the bottom of the valley can easily hear those standing at a higher
altitude rather than on the same altitude and distance) might be
because there will no objects for sound waves to collide with, thus
can travel further
There could be another person quoting the speaker, for example if you observed how the prayer is carried out in some mosques, you'd realise that in some of them some one repeats the Imam with a higher voice to make sure that the rest heard it (For example he may say Allahu Akbar after the Imam has said it)
Not quite sure how many people can fit in historical theatres (when no sound
systems where available), but it would be at least in the
1000s, I'm sure the audience must of heard the show somehow.
References coming soon
One thing to note, is how was this number known in the first place, I doubt any one was in charge of head account.