It depends on your location and venue if you will need external microphone or the built in phone microphone will work just fine.
If you will have a sound system in place (microphone and speakers) in most cases, the phone's microphone will pick up the sound from the speakers.
If your event is in a windy place or outdoors, an external microphone might be needed.
Test Before the Event
Do a test stream before your event to check both video and audio quality.
Watch your test on a computer or another device to make sure the sound is clear.
Refer to our Streaming Microphones Guide for some suggestions if you decide to use an external microphone.