Can I use 2 android phones as walkie talkie? (without internet!) I want to use 2 (or more) android phones as walkie-talkie (i.e. they talk each other without needing internet/voice call).

Of course this should work with WiFi, because it doesn't make much sense a to have Bluetooth or USB connection for this application.
Here is what I use that do not need an internet connection: 

https://play.google.com/store/apps/details?id=com.androidintercom

https://play.google.com/store/apps/details?id=com.remaller.android.wifitalkie_lite