Use APKPure App
Get FMS - Audio delay old version APK for Android
Enjoy a weird experience.
This app plays the sound from the devices microphone through the headphones with a configureable delay.
I was chatting on IRC, talking about app ideas with some friends. This was one of the ideas that made it into an actual app.
It is a weird expierience...
... but that was the intention.
It might not work with bluetooth headsets...