You have 2 options in setting up the Voice Message in K2:
1. You can try replacing your Caller Message and Call Whisper mp3 with regular text. If mp3 is not supported, Twilio will return application error.
2. Alternatively, if you really want to use mp3, you can load the file to any WordPress site in the media files section and use that URL. Aby explained how to do this in Video 2 on the Start Here page of kallzu 2.0. Start watching it from 19:25.