I'll admit up front that I've never coded a read-along book before. I do know how to do media overlays - but I've never had a chance to show off. Opportunities for such projects are woefully rare.
I do like that your post asks for a coder. Most people who want to publish e-books look only for a designer and are then stuck with people who try to build it in MS Word or something and cram the epub or mobi with so much unwanted code that devices often get confused and throw up errors and inconsistencies.
I work entirely on HTML and CSS to design the layout of your ebook, including cover pages, clickable table of contents and illustrations.
In addition to a good coder and designer, you also need someone well versed with image editing softwares if you want to add images to your ebook. It can get a little tricky because the images need to be in a specific size and format so that they look the best and most consistent when viewed on several kinds of readers (say an iPhone versus an HD Kindle).
And if you're going to add audio to the ebook, you also need someone who knows his way around audio editing softwares to be able to get accurate time-stamps and split the audio to fit the text accurately.
It's not very modest of me to say so, but I can do it all. I recently designed (for print and Kindle) a beautiful cookbook with hundreds of images and the experience was great fun. You can see the employer's review on my freelancer profile.
Let me know what you think!