Short answers, pulled from the story.
The Quake soundtrack was composed by Trent Reznor and his band Nine Inch Nails. The idea came from American McGee, who had been listening to their album The Downward Spiral while working on the game. Reznor described the result as textures, ambiences, and whirling machine noises rather than conventional music.
The Quake engine, developed by John Carmack, introduced full real-time 3D rendering, polygonal models instead of prerendered sprites, prerendered lightmaps, and a scripting layer called QuakeC that allowed players to modify the game. Unlike the Doom engine before it, it also offered early support for 3D hardware acceleration through OpenGL.
Romero left id Software after Quake due to creative differences and burnout that built up during development. He opposed several decisions made under crunch, including the shift to a simpler first-person shooter format, and did not receive any money from Quake despite leading the project. The majority of the team, including Romero, Michael Abrash, and Sandy Petersen, resigned after the game's release.
Quake drew its story and setting from a private Dungeons and Dragons campaign run among id Software staff in the early 1990s, with John Carmack as Dungeon Master. A character named Quake, who wielded a magic hammer capable of destroying buildings, was the direct inspiration. The game's gothic and dark fantasy tone was also shaped by the works of H.P. Lovecraft, including the Cthulhu Mythos.
Quake sold 250,000 units at initial shipment, which was strong but not the phenomenon that Doom II had been. During its first twelve months, it sold 373,000 retail copies and earned eighteen million dollars in the United States. By December 1997, the game had sold over 1.4 million copies, and id Software estimated that as many as five million copies may have been in circulation that year.
Quake done Quick is a collaborative speedrun demo in which a group of expert players completed every level of Quake at the Nightmare skill level and edited the footage into a continuous run of 19 minutes and 49 seconds. It was released on the 10th of June 1997. Owners of Quake could replay the demo inside the game engine, watching the run unfold as if they were playing it themselves.