Portable listening devices–now a decade old, and ubiquitous among American children–are being used to listen to audio books, in part shifting comprehension from the written to the spoken word, the Star-Ledger of Newark, N.J., reports. Newark-based Audible.com, the largest provider of downloadable audio books, on March 31 launched its AudibleKids.com division at the North Star Academy in Newark. Dozens of middle-school students got a free Zen Stone Plus MP3 player to download some of the 4,000-plus titles on the new web site. Audible has been very successful: Amazon just bought the company for $300 million. Parents might be the initial core audience for the service, but it’s also drawing interest from schools, teachers, and children’s librarians as a way to encourage kids to read. Listening to literature is one way to reinforce comprehension, experts say…