I have looked almost everywhere trying to find a way to save my android tts output in an audio file. I looked at these posts:
TextToSpeech.synthesizeToFile() doesn't work in real devices. It only works in AVDs.
I have been experimenting with this, too, using both the original Pico TTS engine and a third party TTS engine, trying to write to either the sdcard or the internal memory (on a rooted device):
context.getDir("soundfiles", Context.MODE_WORLD_WRITEABLE);
But, as you noted, the method returns TextToSpeech.SUCCESS without actually creating the file.
If you must record your TTS output to a WAV file, connect the headset output to the aux input in a sound card in your PC and use any recording software to capture that.