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

Fun with Filters

Filters modify a sound by letting some frequency components pass through easier than some others. For example, a low pass filter lets low bass frequencies pass through, but filters out higher frequencies. This is similar to the bass control on a stereo.

In this Applet, you can choose various source signals like sine waves, sawtooth, etc. You can also select different filters and hear the effect of changing their frequency and bandwidth.