What causes the loud squealing sound or distortion you sometimes hear?The major cause of the loud squealing noise you sometimes get from a microphone is feedback - caused when the microphone picks up sound from the speaker, and then sends it back to the speaker, creating a loop of sound.
The solution is not to use the microphone while standing directly in front of the speaker. Experiment with good and bad places to stand in the room before your event! If the position person speaking is fixed, try angling the speakers very slightly to the left or right; that should prevent it.
If the person speaking isn't using the microphone correctly the microphone won't pick up enough signal and the microphone volume on the receiver and/or mixer will need to be turned up loudly to compensate. This can cause distortion, so please try to educate your speakers on the correct way to use a microphone before they begin - there are lots of videos on YouTube which will guide you.
If you do get microphone noise, quickly turn down the microphone volume to stop it.
If you have a mixing desk, turning down the MID or HI EQ on that channel will help to prevent it happening again.