For example the volume register design flaw in the original 6581 chip was used to play back samples. Every time the volume register value was altered, an audible click could be heard. By changing the first 4 bits of the register at $D418 fast enough, samples can be played back with rates up to some kHz (depends on the code and what else uses rastertime) with 4 bit resolution. In the later 8580 model this "bug" was fixed and samples were inaudible, but it is possible to revert this by soldering a 470kΩ resistor or, if you want to be able to adjust the level, a 1MΩ pot in series between EXT IN and GND. Even better sample playback can be achieved by altering the pulse width modulation register, which can hold an eight bit value.