1. A method for using a portable computing device as a synthetic multi-string musical instrument comprising: retrieving a musical score from storage accessible to the portable computing device, the retrieved musical score including temporally coordinated chord and string tracks; presenting string excitation visual cues on a multi-touch sensitive display of the portable computing device in temporal correspondence with the retrieved musical score; capturing a stream of user performance gestures on the multi-touch sensitive display, wherein at least some of the captured gestures are indicative of string excitations; and audibly rendering the user performance on the portable computing device using the captured gesture stream as an input to a digital synthesis of the synthetic multi-string musical instrument executing on the portable computing device with dynamic string retuning in correspondence with chord selections, wherein the captured string excitation indicative gestures, and not the musical score itself, drives the digital synthesis.