When I worked in the OEM industry, there were rigorous standards and compliances that had to be met to ship our phones. If an app was invoking audio recording without the proper permissions, this would be a huge red flag. Google would never approve the phone to be shipped as it would be breaking their CDD.
Also, if audio data was being transmitted using some obscure APN that does not use mobile data or Wi-Fi, OEMs could still easily detect these from the modem side, no matter how encrypted the data is. After all, the app is still JUST an app, in the system folder with all the other apps like Candy Crush. Unless this specific audio recording feature was built into the Android framework, I will say it is 100% impossible.
Note: this is just for Android. I have no idea how iOS works.