If it is hard to hear, people won't listen. When speaking in public you must have an excellent sound system because some of the time you will be speaking while your audience is laughing. Stand-up comics need good sound too, but they are a little different because they tell a joke, then people laugh (they hope). They tell another joke, then people laugh. A good public speaker will be rolling right along making points, showing product features, telling stories, and dropping one-liners and must be heard all the while.
A humorous speech demands a better sound system than a serious speech. In a serious talk, words can be missed and the main message can still be very clear. In humor it doesn't work that way. If key words are missed in a joke or story, it will ruin the humor. No one will laugh and you will look like a giant goober.
The need for a thorough sound check is another good reason to be in the room early. You need to check the microphone to make sure it works. You need to check to see how far your mouth should be from the microphone. You need to know how loudly you should talk. Realize that during your check the audio level should be very loud. People will absorb the sound once they get into the room.
Make sure the sound system is carrying to all parts of the room. If someone is speaking prior to you, try to go to the back of the room to see how he or she is coming across. If you have someone at the presentation with you, have them signal from the back of the room if changes are needed in the public address system after you have started.
If the amplifier controls aren't handy after you have started, you can adjust the sound by changing the distance between your mouth and the microphone and/or increasing or decreasing the loudness of your voice. Try not to use the latter method too often so you don't strain your vocal mechanism.