Richi_S has released a new “Sound System” for A2600 a while ago.

The features are:

– It fits in the Vsync area for all conditions (verified)
– Needs 9 bytes of RAM
– Variable Music Speed.
– It enfeatures one Melodic Instrument with ADSR-Envelope waveform functionalty (AUDF and AUFV are modulated every TV frame)
– It enfeatures one percussion track with up to 30 freely designable Percussion Instruments (AUDC, AUDF and AUV are modulated every TV frame)
– The song can be Programmed with a Tracker.
– In order to save ROM space and coding time, repetitive music data (beats) can be “compressed”.
– Selectable Song Start position, Song End position, “Loop to when song has ended” Position.
– Both Tracks: “Melodic Instrument” and “Percussion Instrument” can be muted with flags separately, thus the game developer can regain control of the TIA for his own sound effects.

http://www.atariage.com/forums/topic/155350-my-new-sound-system/