I have participated in the making of a few 4K intros by programming a softsynth and composing a soundtrack for the intro with it. Here you can find some information about the softsynths and the tunes themselves.

The Ogg Vorbis (.ogg) files can be replayed on Windows using Winamp.

IN4K .ogg
Used in the IN4K Announcement Intro by tonic/Armada, myself and Polaris/Northern Dragons.

The softsynth was done in C and the song data was linked as assembler code since it was easier to organize the byte data as data blocks in assembler code. I tried to create a YM2149ish sound to the tune hence no filters of any kind were used in the softsynth. The synth is able to create three different waveforms: pulse with variable pulse width, saw and noise and has sustain/release volume control. Like all the music editors on the Commodore 64, each individual instrument uses a table for the waveform and note pitch. A new value is read from the table about 50 times per second. With this technique, changing the waveform and note pitch can be used to create more complex sounds like drums.

Released date: Pilgrimage 2005

San Angeles Observation .ogg
Used in the winning 4K intro at Assembly 2004, San Angeles Observation by tonic/Armada and myself.

The softsynth was done completely in C and is based on an earlier softsynth by Ector/Deus Ex Machina. I added pulse width modulation, additional volume control and some other features to the original code.

Released date: Assembly 2004