Note: Background noise issue can simply be solved by reducing the volume of speaker first(then iPhone).
This app allows you to use your phone as a microphone!
It also acts as a decibel monitor and helps you measure how loud a sound is.
You need to connect one end of the AUX Cable to iPhone, and the other end can be connected to a speaker, a PC, or anything that takes input via AUX(headphone port), for it to work as a mic or a voice amplifier! (Bluetooth only supports normal speakers)
It'll work as a real mic, and can be handy at parties, functions, or anytime you need a portable megaphone...
Delay option can be used to echo your voice after the set time... Announce anything! Louder!
Can also be used with an Apple TV, just airplay your phone to it!
Bluetooth supported on iOS 10 onwards! After upgrading to iOS 10, you can use it wirelessly, finally!
Voice may be delayed by less than a second(due to the nature of Bluetooth standards) but will be absolutely instant on AUX.
IMP: AUX Cable must be connected after the app is launched, but before mic switch is turned on...
Note: Reduce volume if you experience noise, or contact me at below email, I'll definitely try to help you out...
Megaphone Microphone Mic Amplifier
No Ads!
I've tried many other apps, this is the only one that works the way I'd like it to, i.e. Amplify my voice using the phone as a microphone thru speakers - Frantic Composure

Great idea to turn your phone into an external mic, it's one less device to worry about! And even better if you're caught in a situation where you need an extra mic! - DixieChickTX
And many more! Mega! Mic Phone! iPhone! PA! Public Announcement!
Note: Background noise issue can simply be solved by reducing the volume of the speaker(not iPhone, which will worsen problem) considerably(though your speech volume will be audible), and voice will be absolutely clear if you reduce speaker volume.
IMP: In rare cases, there may be high noise, to fix it, kindly lower down the volume of speaker considerably and check, contact me if required. Even in low volume, the actual vol. is quite high.