When you play an audio message, after locking the Android phone, the media player appears on the lock screen and it can be played by anyone with physical access to the phone without even knowing the unlock code.
Is there any way to avoid this situation? It also happens in Telegram, but not in WhatsApp.
Thanks and regards 
I had a similar issue. When I started my car, my phone connected to the car and it started to play the last audio source which was a delta chat audio message. I think the issue is related. Perhaps this should be somehow avoided 
I think is the same it is happening to me. Could be a flaw in privacy because gives access to a msg without need of authentication 