That's something I always wondered about. I think it' a gret idea to "spice" up the interactivity of eBooks. Sure, some people prefer plain vanilla text (maybe because that's all they're used to), but why not add features for those of us who wouldn't mind hearing the noises of the bar scene or the playground noises of the kids scene gently wafting in the background as we read the text.
It might bother me to hear the character's voices or see the character's visages if I'd already "created" them in my mind, but maybe not. I used to like the occasional pictures in those old Hardy Boys novels. I think it really added to the story and the suspense. Even with the LOTR books, the illustrations on the paperbacks really set the stage and the tone for the series.
There are purists with everything everywhere, and if all decisions were left up to them we might not have electricity or telephones or cars. Thankfully, progress is unavoidable and if some company takes a chance to make eBooks more interactive and the market responds positively the text-only eBook purists will just have to adapt.