Scheme Note is a Scheme-like music file format and MIDI conversion tool. It takes a text file, which is a representation of a song, loosely tied to a MIDI-like event structure, and outputs a corresponding MIDI file. It is hand-editable for simple songs, and should be very easy to parse for a GUI.
Trommler is an X-based drum machine with the following features: use of 16 bit 44100kHz mono drum samples, realtime audio output using the /dev/dsp device on Linux machines (or audio output to file), a variable number of beats per pattern, volume adjustment of drum samples, stereo panning of (mono) drum samples, and more.
jMusic provides a library of classes for generating and manipulating music, and is a solid framework for computer assisted composition in Java. jMusic supports composers by providing a music data structure based upon note/sound events, and methods for working with that musical data. jMusic can read and write MIDI files, audio files, and its own .jm files. jMusic is designed to be extendible, encouraging you to build upon the functionality of jMusic by programming in Java to create your own music composition tools.
SpiralSynth Modular (or SSM) is an object orientated modular softsynth / sequencer / sampler. Audio or control data can be freely passed between the plugins, and is all treated the same. Data can also be fed back on itself for chaotic effects. LADSPA plugins are supported, and you can use them in the same way as any of the native plugins. Not having MIDI won't restrict what you can do. It currently uses the standard OSS/Free sound input/output (/dev/dsp), and should work on ALSA too. You can also render WAV files directly to disk.
Soundmosaic constructs an approximation of one sound out of small pieces of other sounds. The soundmosaic algorithm is: split the target file up into equal-sized segments, or "tiles". For each tile in the target file, find the closest match in the source files, and replace the target tile with the tile from the source files.
jMax is a visual programming environment for building interactive real-time musical and multimedia applications by allowing the user to interactively design dataflow circuits. The basic data types that can go through are integers, symbols, lists, etc. It is an event-driven system and has been used for MIDI processing. A second part of the system (DSP) allows a continuous signal to flow through a circuit, which is most useful for PCM sound (ie. microphone, sound files, etc.). The system is extensible by using shared libraries, you may add data processor types, data types, GUI elements, device types, and more. Data processors may also be designed as circuits and reused.