Voloco

Voloco is a real-time voice-processing app that combines automatic tuning and vocoding. You can pick a song from your library to sing or hum along to, and Voloco will automatically guess the key of the song and pitch correct your voice to that key. Voloco also lets you record video of yourself singing and share it to Facebook or Instagram

Voloco features four effect presets: Big Chorus -- A seven-voice harmonization effect Hard Tune -- The classic Autotune sound made famous by T-Pain and others Natural Tune -- Gentle automatic tuning SuperVocoder -- A pitched-down monster vocoder

If you hear nothing even when a headset is plugged in, then set More Options --> Settings --> Prevent feedback to OFF.

Voloco compresses your recordings as AAC in an MP4 container so you can easily share them.