SpeechJammer for Android does exactly what the name suggests
http://enrepublic.blogspot.com/2016/01/speech-jammer-for-android-is-super-fun.html
SpeechJammer is a simple app guaranteed to provide you fun
The concept of SpeechJammer is simple. It will playback your voice with some delay so that the listener's brain gets confused and leaving them unable to speak correctly. The key to successful speech jamming is that the volume should be high enough so that the speaker don't hear their own voice (the one that isn't delayed), to insure this make sure the earphones are tightly fit.
You have the option to change the delay. I found out that the default delay of 150 ms works fine and there was no need for changing it.
Apart from the occasional full screen pop up ads, there is nothing to complain about. After all it's completely free and super fun to mess around with.
Download: SpeechJammer