A new tweak has been released in Cydia which will allow you to access to a large music library that is not on your iPhone just like Pandora. The name of this tweak is SimplePlayer. After you have installed this tweak on your jailbroken iPhone it will allow you to browse through top 100 charts for different genres. Simply type in the name of the artist or song into the search bar. This tweak comes with support for multitasking. The music will not stop when you switch to other apps and also using the iPod shortcuts you can control the music.
SimplePlayer tweak is available in Cydia for free of cost. This app is also available in App Store at a price of $0.99 USD. You should note that you will need to have jailbroken iPhone to install this tweak. If you don’t have jailbroken iPhone then you can follow the steps posted here to jailbreak your iPhone on iOS 4.2.1 with redsnow