JSyn - modular synthesis API for Java.
JMSL - Java Music Specification Language
PortAudio - cross platform audio I/O API for 'C'

Audio Examples made using JSyn - a Java Synthesizer API

Multi Segmented Envelope

How it works:

An envelope contains an array of {duration,value} pairs called segments.  These segments describe a contour or shape that can be used to control other unit generators.  In this example, the envelope is connected to the Amplitude of an oscillator.  Portions of an envelope can be queued up for playback. Note that the Synth.FLAG_AUTO_STOP is used to stop the envelope execution when the last segment finishes.  This reduces the CPU utilization which can be seen by clicking on the "Usage" button before and after the envelope finishes.