In this paper, we present our design of MusicScore, a professional grade application on the iOS platform for music composition and live performance tracking, used by composers and amateurs alike. As its foundation, we have designed and implemented a high-quality music engraver, capable of real-time interactive rendering on mobile devices, as well as an intuitive user interface based on multitouch, both built from scratch using Objective-C and Cocoa Touch. To make MusicScore appealing to the general population for their practice and fun, we have introduced a unique auditory capability to MusicScore, so that it can "listen" to and analyze live instrument performance in real time. In order to compensate for the imperfect audio sensing system on mobile devices, we have proposed a collaborative sensing solution to better capture music signals in real time. To maximize the accuracy of live progress tracking and performance evaluation using a mobile device, we have designed a collection of note detection and tempo-based note matching algorithms, using a combination of microphone and accelerometer sensors. Based on our real-world implementation of MusicScore, extensive evaluation results show that MusicScore can achieve acceptably low error ratios, even for music pieces performed by highly inexperienced players.