Sorry, that’s my only theory — volume suppression can be a common side effect of audio session settings, but if it is just about the normal default volume, it’s as loud as it can be without distorting the voices. In my experience it’s about the same as the Google Maps and Siri speech levels so it might be worth checking those out to see if it’s just the general issue that TTS is sometimes a bit on the quiet side in order to play back without additional distortion, or if it is something related to the app settings.