Csound is a powerful and yet easy to use musical synthesis package. Csound was constructed in the tradition of so-called music-N languages, among which the best-known is Music V. It consists of an orchestra- and score-driven executable, written in C for portability. Since Csound is a computational language, it is highly flexible and efficient; complexity is gained only at the expense of computation power. Basically, Csound reads some files and creates the result as a sound file on disk or, on faster machines, in real time through a DAC.
Pysco is a neat utility for CSound. It generates CSound note events and saves to a score file, with unlimited separate output time-streams ("tracks"), unlimited simultaneous different tempos (each track can have its own tempo, including independent accelerandos and ritards), unlimited named "chunks" of events, and many more functions to provid automatic time updating so that the user doesn't have to count beats all the time. It also selects ranges of musical chunks by beat value.
WaveSurfer is a sound visualization/manipulation tool for novice and advanced users, with a simple and intuitive user interface. It can be adapted to different tasks, such as speech research and education, speech/sound analysis, and sound annotation/transcription. You can also make more advanced/specialized applications by extending it with custom plugins or embed WaveSurfer components in other applications. Its flexible interface handles many different file formats, and it runs on many flavors of Windows and Unix. It also supports encoding and Unicode with unlimited file size, and more.
The CMU Midi Toolkit (CMT) is a collection of software for writing interactive MIDI software in C. It includes a number of handy utilities along with an application "shell" that provides timing, scheduling, and MIDI interfaces that are portable across DOS, Mac, SGI, and Amiga platforms.