Hi!
My solution was disabling and clearing cache of google drive.
My story:
I was using my phone with my car's Bluetooth connection very frequently. After one occasion:
watching video on my phone while it was connected with Bluetooth to my car's speakers. (I used like this in the past as well without any error.)
After this I was able to hear call audio only with using Bluetooth devices (either my car's speakers or a Bluetooth headset).
I could resolve the 1st occurrence of this error with a deep power reset (battery+SIM+SD card removal and pushing the power button for 30s).
Than I used my car's Bluetooth again -> no call audio again afterwards.
The power reset was not working anymore.
The official Android 5.0 update just came out yesterday for my country.
I updated the firmware with the official ROM. And the problem gone once again. At least I though after testing the call audio with one call.
But today it was not working again.
So I tried everything from this forum and the disabling and clearing cache of Google drive resolved my problem.
It shouldn't have anything to do with Bluetooth, though I could start at least 4 calls with hearing in call audio once again.
Some other things I tried without success: reconfiguring audio routing with SoundAbout app, "OK google" disabling, S voice disabling, lint removal, removing paired Bluetooth devices, different shutdown sequence when connected to my car's Bluetooth, toggling airplane mode.
I lost all of my faith to Samsung. The in call audio is the essential feature of the phone, such an error should not happen any time.
Good luck to find your solution for the same issue!