Microphone App! Convert your iPhone into a live PA system using AUX Cable or using Lightening Cable converter to connect it to an AUX Speaker.
Also has option of delay, which repeats what you say after set time.
From public Announcement system to house parties, you got everything covered...
Note: This is wired microphone, and NOT bluetooth/wireless one... Checkout other app for that facility.
This app has been updated by Apple to display the Apple Watch app icon.
Microphone MegaMic Megaphone 
Ding Dong Bell AUX Bluetooth 