Trap Studio (US$2.99) is a music creation app that includes more than 350 samples created by Prime Loops. Along with that, the app features dual Oxford Synthesizer Company (OSC) Synthesizers, and you can import samples and apply many effects to them to create a unique sound. Sounds from Prime Loops have been featured in chart-topping tracks from artists such as Skrillex, Lady Gaga, Beyonce, and Grammy Award winning producers including RedOne & DJ Frank E.
Samples are categorized as Drums, Bass, Synth and SFX, and you can apply effects like distortion, echo, flanger and more to your samples as well. There is a virtual keyboard so you can enter your own compositions. Use your iOS device built-in microphone to add vocals or other audio, and then saved tracks can be uploaded to SoundCloud or shared via email, Facebook, Twitter, Tumblr and Foursquare. Files are saved in uncompressed .wav format and Trap Studio includes complete documentation.
I tried creating some background tracks and it was pretty easy to do even without looking at the help files. You can overlay tracks, of course, and edit their in and out time. I was able to save a project and come back to it later for additional work without any glitches.