This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
All Apple II computers can produce "Old Apple" sound without any special boards or add-ons. The system BEEP you hear upon a Reset is an example. The sound is produced by code which addresses memory location $C030. Each time $C030 is referenced the output of a flip-flop going to a simple audio amplfier stage changes state. For example, in BASIC, X= PEEK(49200) will produce a single click. In machine code, so would AD 30 C0 (Load Accum from address $C030). Duration and Frequency of sound can be set by placing the address reference instruction inside a loop and using other loops to control speed of references. Although the signal going to the internal amplifier is always a square wave, creative coding by music enthusiasts, game developers, and other users has produced remarkable effects ranging from two-voiced music and game sounds to speech. ____ By: Mitch Spector