If you're using Pandora, you need to uncheck "Conserve battery option" option under the Menu > Settings > Advanced. Now the screen will stay on while the music is playing on Pandora.

You can also download the apps like [KeepScreen][1] and [Stay Alive][2] to block screen timeout when predefined applications are active.


  [1]: https://play.google.com/store/apps/details?id=com.tni.KeepScreenLite
  [2]: https://play.google.com/store/apps/details?id=com.synetics.stay.alive&hl=en