Please try to verify the following on Talkatone settings:
1.- Tap on Miscellaneous option
2.- Tap on Call Settings option
3.- Make sure that the Native Audio option is Off.
4.- Set up the Audio Playback Stream option for Voice Call.
5.- And the Solo Audio Stream option is unchecked
Increase the audio (using the buttons on the side of your device).
Then restart the app and try again.
You can restart the application by following these instructions:
Tap on the Quit option under Talkatone settings (open Talkatone Settings by tapping on the Android menu button). You will find the Quit option at the very end of the options list on the screen.