You want a hard reset. Hold down the outermost buttons and press the soft reset button. The screen will start clearing. Wait a minute or two, and then reset again. Whammo! It's empty.
(The usual warning - sync everything first - applies, of course - you'll lose all the data on your device, otherwise.)
By the way, if you want MP3's, you should really just consider getting an SD card instead. They're not that expensive at lower capacities.
--janak