eInk Readers are for people who want to read. They don't play Angry Birds, They don't check eMail while you read. They don't tell you you have a new facebook message. They don't ping, bleep, ding when someone else sends you a text/eMail, Facebook message. In fact, they just let you get on with reading without telling you your friend has made his/her move in Words With Friends. This multimedia is going to get in the way of reading. You'll be using a tablet for those sort of eBooks and you will get distracted.
It's all about the words. The videos or audio don't need to be there if the words are good enough to convey the meaning. In fact, having video can ruin the reading experience. How many times have you seen a movie and then read the book the movies was based on only to have the cast of the move stuck in your head instead of your imagination deciding for you?
Even if we are able to ignore the multimedia in the eBooks and just read, we'll still have the bloat of the multimedia in our eBooks. And we won't know which eBooks with multimedia work well without activating the multimedia. I can see the words dying because there's a video to show you what the words should have been telling you.
Multimedia in eBooks could be a huge step backwards. And I see it as something that's a gimmick and I see it being overused and ruining reading.