Screenshot by Seth Rosenblatt/CNET

It's a fallacy of the superhero genre that a new costume accompanies every power boost the hero earns. That's also the case with a major upgrade to Marvel Unlimited, the preferred (and only) subscription-based comics reading app for fans of Spider-Man, Iron Man, and Captain America.

Scheduled to be announced on Sunday afternoon's Marvel panel at South by Southwest Interactive in Austin, Texas, and currently live in Apple's iTunes Store, the new version of Marvel Unlimited looks a lot like the old version of Marvel Unlimited. Don't let that fool you, though, as the changes could present a way forward for smarter integration between traditional comics and multimedia features.

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A year in the making, the new version of the app comes with three important changes: transitions from one panel to the next are faster and smoother, there's now adaptive audio, and Marvel AR's DVD-style extras are now included in its digital comics for the first time.

While the added features might turn off more traditionalist comics readers, Vincent said that the goal was to make digital comics feel more digital without losing their essential nature.

"We knew we had something conceptually," she said of last year's adaptive audio announcement by Marvel at SXSW, "but we weren't sure how it was going to work with comics. We wanted to figure out how we could tell stories in new ways."

Arune Singh, executive director of TV Communications at Marvel, described the change as one that could alter how the company makes its comics available electronically.

"The way we used to paste comics together on the page in production, so far we have been pasting comics on our iPad. Now," he said, "we have adaptive audio."

Screenshot by Seth Rosenblatt/CNET

The features can also be disabled in the Options panel. Because Marvel Unlimited is a subscription-based app, the features have been added at no extra cost. To entice new readers, Marvel has dropped the cost of subscribing for the first month to $0.99, if you sign up by March 14 with the promo code SXSW99. Normally and thereafter, Marvel Unlimited costs $9.99 per month, or $69.99 per year.

The overhaul to the comics reader has swapped out HTML5 for native iOS code, so page turning and panel transitions are smoother. The update to the Android app is "so, so close" said Kristin Vincent, Marvel's vice president of digital products. Both were done as work-for-hire by Chaotic Moon, Marvel's long-standing partner on Marvel Unlimited.

The big new feature, though, is that Marvel is finally making available to its readers what it calls "adaptive audio." It's similar to a background soundtrack or movie score, but it detects when you move from one panel to the next and plays themed music appropriate to each panel. It can also gauge how quickly the reader is changing panels, and will adapt the score to suit the pace of the reader, says Marvel.

The music plays from when the comic loads, starting with the book's cover, and then adding in adaptive elements such as the pinging sonar of a submarine when viewing a panel of a submarine and mines.

The adaptive audio, which is only available on Captain America: The Winter Soldier storyline at first, probably owing to the extra time required to create the score for each comic. Vincent said that other comics will be added, but added that Marvel wants to hear from its fans on what they think of the debut.

Screenshot by Seth Rosenblatt/CNET

"I think we have the best fans in the world, and the most vocal fans in the world. We're always watching the metrics," she said. "We look for what people are sharing in the social space. We're very in tune with our fans, or at least try to be, and move where they move."

Marvel's previous partners on the adaptive audio, CORD and Momentum, are no longer working on the project. In their place, Marvel has signed on Melbourne, Australia-based Firelight Technologies' FMOD, which also is used in video games such as Crysis, Bioshock and Guitar Hero, and contracted with Emmy-nominated composer David Ari Leon's SoundMind Music. SoundMind is no stranger to Marvel, having also worked on Marvel cartoons and motion comics.

Marvel AR, or augmented reality, videos have been available previously only to people who bought the printed comic. You would then have to hold your phone or tablet over an AR-enabled panel to activate the video, and it would show you a behind-the-scenes look at the comic, often interviews with writers or artists, or the process of developing the page.

Marvel has begun integrating that AR content into Marvel Unlimited with 20 videos related to the first issue of author Jonathan Hickman's Avengers series from 2012. The company plans to move its current 1,400 videos to Marvel Unlimited, Vincent said, as well as adding new videos.