With Shout, you can convert your smart phone to a microphone and use it with any speaker system via audio jack or Bluetooth. Well, there may be other apps like this in Play Store. What makes Shout different from other similar apps (if available) is something I leave to your experience. With highly optimized audio library, Shout offers stunning clarity and zero delay (best experience while using audio jack). Try for yourself.
PS: Take care not to use the App while directly in front of speakers. This will cause an Acoustic feedback, the same way like when you use a normal mic in front of speaker. Keep your speaker in front facing audience and stay well behind the speakers. Happy Shouting!!!