With the new iPhone operating system now out (iOS 4), expect to see a host of updates to existing music apps, as well as new apps flooding the market. They include:

UPDATED (6/24):
- NPR: The NPR Music app offers footage of live performances interviews and exclusive first listens to unreleased albums, as well as live-station streaming and the ability to play the station in the background while performing other tasks on the phone.

- Smule: The company introduced a new version of the popular I Am T-Pain app that allows users to use the front-facing camera to record themselves singing along to the Auto-Tuned T-Pain tracks, and share the video. T-Pain himself demos the new capabilities, while others try it out here.

- Slacker: Supports multitasking so music from the personalized Internet radio service can be played in the background as users do other tasks on their phones.

- Pandora: Supports background listening, so users can stream Pandora while simultaneously surfing the web or checking e-mail.

- Sonos: In August, Sonos will make available a new app to turn an iPad running iOS4 into a remote control for its ZonePlayer multi-room wireless music streaming service. The app will let users see what music is being played in which room through the system, and make changes to content, volume and playback.

Stay tuned to Billboard.biz for more iOS 4 app updates. And app developers: submit your smartphone apps for Billboard’s Music App Awards now. Deadline is July 30!

Questions? Comments? Let us know: @billboardbiz