Here are 2 websites that offer audio comic books and I'd love to have as apps: http://www.graphicaudio.net and http://www.comicsempower.com
I don't think you can download the comics you buy to Apple devices. The latter site is geared toward us and they have a writing contest going on til March and check out their first-timers guide to comic books. You can find a Batman or Spider-Man audio comicbook on youtube if you type "Graphic Audio" into the search engine.
By BigMurph, 1 January, 2016
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you can, in fact, get them into apple devices
graphic audio has it's own app for apple devices. However, I find it clunky. Thhe best way I have found to interact with their stuff is to download the books in zip file format, and put them in dropbox, where VoiceDream can load them. In theory, that process should work for the other cite. So these books are easily transported to apple's ecosystem.