“Aligning” seems a perfectly good word to me. It’s the technical term that gets used, and it’s not particularly overloaded so people aren’t going to expect it to mean something else, and it’s easy to demonstrate even pictorially.
(I would also use the term “synchronising” rather than “syncing”, because “syncing” has kinda developed a meaning of its own, even though it came from the same place. And use the word “with”, and try to use the words “text”. “Synchronising ebook text with audiobooks” is way better than “syncing ebooks and audiobooks”.)
(I would also use the term “synchronising” rather than “syncing”, because “syncing” has kinda developed a meaning of its own, even though it came from the same place. And use the word “with”, and try to use the words “text”. “Synchronising ebook text with audiobooks” is way better than “syncing ebooks and audiobooks”.)