Short answers, pulled from the story.
The first programmable music sequencer was an automatic flute-playing machine built by the Banu Musa brothers in 9th-century Persia, described in their Book of Ingenious Devices. It was powered by steam and hydraulics. The brothers also built a hydropowered organ using exchangeable cylinders with pins.
Al-Jazari, an Arab engineer, invented the first programmable drum machine in 1206. It was part of a robot band of four automaton musicians, two of whom were drummers. The rhythms were changed by repositioning pegs, or cams, that triggered percussion levers.
Raymond Scott, an American composer of electronic music, invented the first electronic sequencer using thyratrons and relays. During the 1940s and 1950s he also built the Wall of Sound, an electro-mechanical sequencer installed in his New York studio, and in 1959 completed the Circle Machine, which generated rhythms optically using a rotating photocell and incandescent bulbs.
MIDI, Musical Instrument Digital Interface, is a standard protocol for communication between musical instruments and computers. It was unveiled in 1983 by Roland Corporation founder Ikutaro Kakehashi and Sequential Circuits president Dave Smith after negotiations that began in June 1981. MIDI has remained the musical instrument industry standard interface since its introduction.
Released in 1977, the Roland MC-8 MicroComposer was the first stand-alone, microprocessor-based polyphonic CV/gate sequencer. It supported eight-channel polyphony and held up to 5,200 notes in 16 KB of RAM. The MC-8 and its descendants, including the Roland MC-4, had more influence on popular electronic music production in the 1970s and 1980s than any other sequencer family, with Yellow Magic Orchestra among its earliest known users in 1978.
The first computer music piece was performed in June 1951 on CSIRAC, Australia's first digital computer. The piece was Colonel Bogey. In 1957, Max Mathews at Bell Labs wrote MUSIC, the first widely used program for sound generation, and a 17-second composition was performed on an IBM 704 computer.