Why Music has become such an important part of online slots
In the 1930s the Mills Novelty Company built a slot machine quiet enough that players nicknamed it the silent bell, which tells you what the rest of the floor sounded like. For most of the machine's first century its sound was whatever the mechanism produced: the clack of the reels, the bell that rang on a winning line, coins landing in a metal tray.
When the sound got a composer
That changed when the machines became software, because software can play anything. The early digital games looped a jingle. The current ones ship with scores written to the mechanics, and the writing borrows two techniques from video games. Vertical layering keeps several stems of the same cue running and adds or removes them as the state changes, so a base-game loop gains a drum track when a bonus symbol lands and drops it when the spin resolves. Horizontal re-sequencing lets the cue jump to a different section when the game moves into a feature, so the music at the start of a bonus round is a chorus arriving on cue, with the transition written to land on a bar line so the jump is not heard as a cut. A win of any size gets a stinger, a rising figure timed to the counter.
Researchers have measured what that does to a player. In 2014 psychologists at the University of Waterloo, led by Mike Dixon, had 96 gamblers play a nine-line slot simulator with and without winning sounds. Skin conductance, the standard lab measure of arousal, was significantly higher with the sound on, and 72.5 percent of the players preferred it that way. Studios hire composers specifically for that response.
The catalogs arrive
Once the audio engine could carry a real song, the bands followed. NetEnt put out three rock titles across 2016, Guns N' Roses in January, Jimi Hendrix in April and Motörhead in September, and the first of them let the player choose the track from a five-song setlist: Welcome to the Jungle, Sweet Child o' Mine, Paradise City, November Rain and Chinese Democracy. Play'n GO built a whole music series, the earliest entry being Sabaton in May 2019, "created in partnership with the legendary Swedish power metal band," and it now runs through Twisted Sister, Alice Cooper and, by the studio's own description, *NSYNC. The studio releases the soundtracks as albums on a site of its own, with the game's cues sequenced as tracks.
Def Leppard's Hysteria joined the series in March 2022, built on a grid instead of reels, and among online slot games it is the clearest case of the music and the mechanic being designed together: the studio's page says the feature named after Pour Some Sugar on Me "visually syncs with how the symbols act on the grid, dropping to the bottom of the grid as if pouring sugar."
The band gets a licensing line on a 1987 album and an audience that will hear the hook more often than any radio station plays it, and the studio gets a theme it did not have to invent, along with artwork and a fan base that already knows the words.
Mixing as a design discipline
The last piece is the mix itself. On a phone, most of this plays through a single small speaker or a pair of earbuds, and the sound designer has to make a stinger cut through a loop at the volume of a commute. The loop also has to survive being heard for an hour, which is why so many are built on a bed that changes slowly under a stinger that does not. Loudness has to be managed as well, since a stinger mixed hot enough to cut through a bus will clip in earbuds. On the casino floor the industry went the other way. The Michael Jackson cabinet that Bally Technologies unveiled in 2011, on floors from 2012, put the audio into the seat, a surround-sound chair that vibrated with the music.
Anyone reviewing a live show listens to the mix before the lights, and a slot's audio is doing comparable work through a phone speaker, mixed by someone who thought about it just as hard.