You can find several dedicated text-to-speech synthesizers that offer tools to create an audio (like .mp3 file) with a speech from a saved text file, or document but they are usually pretty expensive. And even that will in most cases require manual tweaking during the process because otherwise the audio would not be perfect. However, you most likely won't find a simple docx to mp3 conversion that does the job with a single click.
Open DOCX file Open MP3 file