Twitter has built a new card for playing audio inside tweets, and its first partner is a company it once considered buying. Soundcloud will bring its full catalog to Twitter Audio Cards, the companies said today, the first of many partners to come. And the audio card has a unique function: you can "dock" the card inside the Twitter apps for iOS and Android so that you can keep listening as you scroll through your timeline.

You've been able to embed audio inside tweets for a while, but Twitter says its new card is more advanced. You can start playing a song with a single tap. Cards display album art at full screen or minimize it to keep reading Twitter while they listen. It's a logical way of bringing new media into Twitter's text-heavy screen, but you wonder how often people will use it. Many Twitter sessions on mobile devices don't even last as long as a single song. Still, for artists looking to promote their work, the audio card on Twitter will offer a useful means of self-promotion.

Soundcloud says it's launching the ability to create cards with "a small group of music and audio creators," including This American Life, NASA, David Guetta, and Steve Aoki. The feature will come to everyone on Soundcloud over time, the company said.

Update, 5:40 p.m. Strangely, less than an hour after making its initial announcement, Twitter posted a new item on its blog saying iTunes is also a partner on the new audio cards. Foo Fighters announced the release of their new single using the feature:

As of today, the Foo Fighters' tweet is the only one that displays an iTunes link in a card, but more artists will be added over time, Twitter says.