...being loud enough is another. It all depends on your audience--who they are, how many, how far away the person farthest away is, and where they are.
There have been many times that I've been in a large group of people and the microphone that was being used suddenly doesn't work right (TDO=technical difficulties occur). Rather than wait for the microphone to click back to life, a person says, "I don't need a microphone; I'm loud." Unfortunately, there's more to being heard than being loud. In theatre, it's
Projection is being loud enough to be clearly heard by the person in the very back of the room. It's not yelling but it is "throwing" your voice against the back wall.
And here are the other things to consider:
*Who your audience is: seniors? family groups? mixed group of adults? You've got to take into consideration if most of the people in your audience will have a hard time hearing you. Senior citizens might need someone to be louder. Audiences made up of families will need loud, too, because sure as you begin speaking, a child is going to start talking (or whining). The group of adults might think you're no longer speaking to them and start talking to their neighbor. It happens. Know your audience so you can project to be heard.
*How many audience members there are: Projecting for a group of 20 is going to be different than projecting for a group of 100. The more people there are, the more energy needs to go into projecting.
*The person in the very back: For awhile we stopped attending a high school's theatre productions--and we loved going--because we usually ended up in the back and the projection just wasn't good enough. Why go if you can't hear? (The high school built a new auditorium; we're attending plays again). Those in the back are important, too. A hundred people in a perfectly square room is different from 100 people in a long, rectangular room, which is different than 100 people spread out outside. Use the microphone if you know your voice won't reach the person in the very back.
*And where's this audience? If the audience is in a theatre, more than likely there will be some pretty good acoustics--sound carries easily to the person in the back row. Mostly, I've seen those microphones in classrooms, large banquet halls...well, everywhere but a well-designed theatre. Without a microphone, the voice may not carry well. And if you're outside, sound waves seem to go every which way. Also, if you're outside, there's always a truck that rolls by just as you start to speak without a microphone, or an emergency vehicle passes nearby, or the wind whips up. Unless you're standing in the Mormon Tabernacle (which has incredible acoustics), use a microphone.
So it comes down to this:
*Use a microphone if you've not practiced projecting.
*If you can't use the mic or one's not available, don't just be loud, project your voice to the person in the back.
Last little tip: Projection is filling a room with sound--you can "feel" it. If you have to ask, "Can you all hear me?" more than likely they can't. The people who answer "yes" are usually sitting in the first few rows. Be sure to pay attention to the blank stares in the back; those are the people who didn't hear your question in the first place.
And another last little tip: If the mic goes, get closer to your audience or have your audience get closer to you.