As an audio engineer, I have always cringed a bit when I hear books with built in sound. The sound quality is always going to be terrible on tiny speakers, and they are often shrill and hard to understand. I also think some imagination is lost – just like watching a movie before reading a book. There’s a lot of creativity in letting your mind come up with its idea (in sound and picture) of what a character looks like, or what they sound like.
When done well, sound can enhance the experience of reading a book. Electronic reader sets allow kids to interact with books in a way that sets them up to play/read on their own (knowing when to turn the page, figuring out what page to play next based on a symbol). While these still have small speakers, the sound quality has never bothered me!
In terms of sound quality, I am also a fan of e-books with sound, even on mobile devices. The sound quality on an iPad is going to be far better than a cheap speaker built into a book.
As with any device for children, you also have to be conscious of volume. Toddlers put everything up to their ears, and something that sounds too loud at a short distance is even louder up at your ears. Putting tape over speakers is always an option, but in the case of books, could make it harder to understand.