IN UTERO!!! Science tells us that sounds and voice familiarity begin in utero. It is a logical conclusion that language development can be enhanced by early exposure to reading. There was a famous cellist that believed his ability to know the next page of an "unfamiliar" piece of music was due to his mother practicing this exact composition during her last trimester. I can tell you from personal experience that we read to both our babies a lot (in both french and english) before our children were born. They both ended up at Harvard and are considered gifted at language so I'm sticking by this belief that early reading is beneficial to educational outcomes. In any case, reading is calming and your baby will hear and eventually identify your voice. So I don't see a downside to this "experiment".
Here is an interesting article I found on pre natal brain stimulation: https://www.pnas.org/content/110/37/15145