A previous answer to a similar question may be helpful:

Before the advent of speakers and microphones, did people speaking publicly in front of crowds have to yell?

Also, to anyone wondering what "Habemus Papam" means, it means "We have a Pope."

After each round of elections the ballots were burned in a stove with a chimney projecting above the Sistine chapel. If no pope had been selected, wet straw was included, resulting in black smoke. If a pope had been selected (Habemas Papam) the smoke would be white.

This is still done, although chemicals are added to ensure the colour difference is obvious.