There's an app called beatmaker intua
http://www.intua.net/products.html, it's a sequencer/sampler/dum machine all in one. You can download or upload yr own audio files and the possibilities are endless. I've had the thing for a day and made 4 songs already. All of them suck.
It's $20 and worth ever penny. I spent $300 on a shitty sampler 5 years ago that can't do 1/4 of what this can. Get it