Why does my screen blink on and off when I play a WhatsApp voice note?

[Android Central Question]

I have a galaxy s4. I changed my regular call settings on settings that the screen should stay on during calls. That works when using the phone. But When I use WhatsApp or textnow, the screen shuts off as soon as I begin playing a voice note or making a call, and i cannot use the screen even to select speaker. I would like to use the phone while listening to a message, but the phone goes totally our of commission until the message is finished. I have no option of pausing the message, cause the screen goes off (sometimes blinks on off). I can only use the power or home buttons as a saviour.
How can i change this?
Thank you everyone in advance