When generating the audio, TTS just speaks a word or two. It is like the timestamps hold it. And the mp3 audio which is generated has full length. I am only able to listen to the audio of the entire text if I download the file.
Sorry to hear that. We will investigate the issue.
As far as I understand, sometimes there could be issues with generated audio for large lessons when certain TTS voices are used.
Can I ask which TTS voice you are using? (this can be seen in Settings → Reader → Text-to-speech Voice)