“Want to keep mobile users engaged longer?” I wrote a year ago. “Just add audio.”

Flipboard, the popular “social magazine” for the iPad and iPhone, already enjoys high user engagement. As of January the app has been downloaded eight million times, the company says, and users spend an average of 90 minutes per month with it. (It’s no Globo, but that’s a solid number.)

Now Flipboard’s smorgasbord of lean-back content gets a big addition: audio. This week’s update brings in content from NPR, PRI, and a host of independent podcast producers by way of the social audio platform SoundCloud. And if past data is any indication, audio could keep users “flipping” a lot longer.

Most users of NPR’s mobile apps never listen to audio, but those who do consume twice as much content as people only reading text. In other words, users who play an audio story are likely to read more articles. On average, according to data from January–mid-April 2011, audio streamers racked up 4.2 pageviews per visit versus 2.4 for the text-only crowd. Same ratio for iPad users: Listeners viewed 8.1 pages per visit, while readers view 3.9 pages. (I have asked NPR for updated numbers; watch this space.)

It makes sense: Audio is the perfect multitasking medium. In the updated Flipboard app, I can hit play on the latest episode of The Dinner Party and continue flipping through articles. Why wouldn’t I want an aural accompaniment?

The partnership fits with NPR CEO Gary Knell’s vision of “radio everywhere,” as public radio braces for a future beyond the terrestrial broadcast. (See also: Infinite Player, the connected car.)

And because Flipboard promotes discovery of new content, with channels that can be customized to users’ tastes, lesser-known productions may get more attention. When I select the “Audio” tab on Flipboard, I see Roman Mars’ independent program 99% Invisible next to public-radio powerhouse Fresh Air. I also see Snoop Dogg, one of SoundCloud’s featured users.

SoundCloud has spent the past year investing in partnerships with public radio and other providers of non-music content. The company wants to be for audio what YouTube is for video: part hard drive in the sky, part community, the place everyone goes to upload sound when they need to embed or share it.

In a statement, Flipboard founder Mike McCue said the addition of SoundCloud is an example of “how Flipboard makes social content a more discoverable and more immersive experience.”

It’s also an example of Flipboard’s more-is-more approach to serving users, who have come to expect an overabundance of choice.