There was a song in Chrono Trigger that didn't make its way into the game, but fans discovered it through the soundtrack and a leaked build that made its way online. It took over a decade, but the song finally made its way into the game through the DS port.
Chrono Trigger has one of the best soundtracks of all time. This is a soundtrack with songs like "Corridors of Time", "Yearnings of the Wind", and "Magus Confronted", all of which are considered to be classics. The soundtrack was composed by Yasunori Mitsuda and Nobuo Uematsu, and it stands as some of their greatest work of all time.
The Mystery Of "Singing Mountain"
Square Enix would often release the soundtracks for its games on CD or vinyl. It didn't take long for the fans to notice that one of the songs on the Chrono Trigger soundtrack wasn't actually in the game. The song was called "Singing Mountain" and it was the 24th song on the soundtrack, between "Magus Confronted" and "Tyranno Lair." The song's placement on the soundtrack led fans to believe that it was meant to be used in the prehistoric era of Chrono Trigger, after the party is accidentally sent back in time during the battle with Magus.
The mystery behind "Singing Mountain" was cleared up in an interview with Mitsuda. According to the Chrono Wiki, Mitsuda explained that "Singing Mountain" was meant for a dungeon that was cut from the final version of the game, as it didn't contain any plot-relevant problems that needed solving.
The Leaked Demo Build
Fans were given a glimpse of the original plans for "Singing Mountain" years later. Square Enix has admitted that it never kept the source codes for its old games, so people expecting Nintendo Gigaleak content for old Final Fantasy games are out of luck. Chrono Trigger is the exception to this, as a demo cartridge was sent to Japanese gaming magazines a few months before it was released. The demo cartridge was later uploaded to the Internet and fans datamined its content. The Cutting Room Floor contains a detailed breakdown of the altered content in the demo.
The most interesting piece of cut content in the demo build is an incomplete version of the Singing Mountain dungeon, which includes the song as its background theme. Singing Mountain was a dungeon from the prehistoric era that used similar visuals to Heckran's Cave, with the water being turned into lava. Singing Mountain lacks any enemies or treasure chests, but it does contain a large open area, a map full of small caves, and a final walkway area that would likely have housed a boss battle. These maps were replaced in the final version of the game with the ones used for the Black Omen, so fans would never have seen them if it weren't for the leaked demo.
The Song Returns In Chrono Trigger DS
Chrono Trigger received a port to the Nintendo DS in 2008, which included brand new content. Once the player completes the game, they will unlock a new post-story dungeon, called the Dimensional Vortex. This dungeon has a new area called the Frozen Cliffs, which uses "Singing Mountain" as its theme. It took over a decade, but "Singing Mountain" finally made its official appearance in Chrono Trigger, even if the dungeon it was created for didn't make it in.
Source: The Cutting Room Floor, Chrono Wiki