How to change the default media player on iPhone?
There is no current way to make Pandora Downloader the default music app on iPhone (even jailbroken). There is a way to make Spotify the default music player (see here), but all that tweak does is change the little icon in the multitasking bar. When pressing play the music app still plays. While maybe that is a technical issue, there is also the functional problem of what the phone should do when the button is pressed. Should it play a random song? The first song in a playlist? It's unclear.