usb device - Simple Audio Sampler And Looper

I am seeking to build a small drum pad that can trigger short mp3 files. Here there are a lot of specifications and I don't expect to have everything explained, but any tips on how to go about making this, what IC's to use especially, or any general help and explanation would be greatly appreciated,Thank You. -Kevin

Basically the goal is to have eight buttons that will each play a short mp3 clip when pressed.

More things that would be good are:

Press more than one at the same time

A bank selector switch that lets you chose between different sets of mp3's that will be played when each pad is pressed.

The ability to download mp3s to each pad and change what each pad will play.

A Looper to let you record short loops, play them back, and overdub (record over the last recording)

Rechargeable battery (via USB) with low battery LED indicator.

Again, I realize that these are ideals, and also that it might be easier to create a custom IC to fit the needs (If anyone knows about where and how I could do that please point me in that direction by the way). But please give me some general direction or outline of how I should go about this, and any tips that might be helpful. Here is my best attempt at a general schematic of my idea. Feel free to dismiss if it is incorrect or unfeasible. Thanks again!

Update: I found a great chip that I think might work for my purposes,the ATTINY861-20pu. I found a project that Plays WAV files off of an SD card. Any Ideas on how to better integrate this chip with the features above? Heres the Project: http://elm-chan.org/works/sd20p/report.html

Update 2: Drew out a block diagram for a circuit with the Atiny 861, I'm just encoding each pad to binary to control the triggers on the Atiny, whitch plays .wav files from the SD card to the Headphone Amp(LM4910). The bank switch just shifts the binary number up to where I will place a different set of samples on the SD card to be accessed. ----- Problems: Can't press more than one pad at once? How do I load the firmware to the AVR? Will the higher powered PWM output ruin the Headphone amp? Haven't gotten to the looper part yet. Any help or criticism greatly appreciated!

^{simulate this circuit – Schematic created using CircuitLab}