This old 24-pin dot matrix printer has been converted into a MIDI compatible sound generator using an ATmega8 and a Xilinx FPGA. Up to 21 notes can be played simultaneously (16 MIDI channels with individual volume and pitch). The original printing frequency was approximately 1kHz with a pulse width of 300μs — pins hit the paper at a maximum of 1000 times per second during printing. The MIDI electronics increases this from a few Hz up to 2kHz. When the pulse width is reduced the sound gets quieter because the pin hits the paper with less force; see the full technical details.
(also see what can be done by manipulating floppy disk drives, especially for Super Mario)