I would not trust anything on a phone as being secure. The phone ecosystems these days are all designed around data collection so even if Whatsapp itself is secure, you don't know what's going on with the data (ex: your voice) before it even reaches the application and gets encrypted. Facebook, Google, etc are more than likely listening to your voice and generating metadata to sell to advertisers. I have seen plenty of times where relevant ads pop up based on IRL conversations or actions. Even if you somehow disable the mic, the accelerometer is actually sensitive enough to pickup voice through vibrations.
If you're looking for secure voice chat I would look at some kind of voip setup that is self hosted, and then having the traffic go through an OpenVPN tunnel. Only allow IPs you trust to connect to the OpenVPN server. (let's not forget heartbleed - there could potentially be other exploits like that which are not discovered yet)