In professional audio, the transition from analog to digital technology is nearly complete. The more recent shift towards systems based on general-purpose computers has been gradual, because such computers lack the processing and I/O performance needed for professional audio applications. The SonicSystem, a professional digital audio system, solves these problems using DSPs and a specialized file system. We describe the uses and implementation of the SonicSystem. A distributed version of the SonicSystem, using FDDI networking, was released in 1992. It provides transparent real-time access to remote sound files, and can handle 100 or more CD-rate channels simultaneously. The system provides guaranteed performance: an activity (such as playing a sound file) is permitted to start only if it can be assured of completing with no dropouts. These properties are achieved by the use of a special-purpose transport protocol, real-time scheduling, and resource reservation.