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Sonic Part 2 - Did You Know Gaming? Feat. WeeklyTubeShow
Sonic - Did You Know Gaming? Feat. WeeklyTubeShow
 
Despite most of Sonic CD's soundtrack being replaced for the game's American release, all of the past versions of the level themes remain unchanged from the Japanese/European originals. This is because unlike the rest of the game's soundtrack, in which each song is encoded in the Mixed Mode Compact Disc Digital Audio format (more commonly known as Redbook audio), the "past" themes are encoded using pulse-code modulation, making them much harder to switch out.
Contributed by game4brains
In the 2011 remake, entering the code PCM32 DA8 into the sound test brings up a level select that allows the player to start any level as Tails.
Contributed by 不明?
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In the Japanese manual for Sonic CD, Amy Rose is referred to as "Rosie the Rascal".
In the U.S. Manual, her name is listed as Princess Sally.

There are theories that her name was changed to Princess Sally in the U.S. manual of Sonic CD to tie the game to the Archie Comics, or the Saturday morning cartoon.
Contributed by ABOhiccups
In the Sonic Gems Collection version of Sonic CD, the water in the game is clear. That is because it's a direct port of the PC version, which utilized a specific graphics card for the water which wasn't emulated.
Contributed by xaxdxaxmx
In the 2011 re-release, there was originally going to be an extra boss that would have only been accessible after the player had collected all of the Time Stones, but it was ultimately scrapped.
Sonic CD's JP/EU intro song "You Can Do Anything/Toot Toot Sonic Warrior" is actually a remix of the theme of Green Hill Zone from the Game Gear/Master System version of Sonic the Hedgehog 2.
Contributed by Takahashi2212
The 2011 rerelease version includes both the International and Japanese version's music. The user can toggle between the two music versions. However, due to licensing issues, some vocals from the Japanese soundtrack have been removed.
Contributed by Funland47
The original Sonic CD intro is shorter and slower than in the updated versions. That includes collections and the HD version. The original includes scene of Eggman's face on the mountains in the original, but was cut on the updated versions.
Contributed by Funland47
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When the code FM40 PCM12 DA11 is inputted in the unlocked sound test menu, a screen with Tails next to a car saying "See you next game!" will appear.
Contributed by Funland47
The game's CD also serves as its soundtrack - inserting it into a normal CD player plays the game's music.
Contributed by Funland47
The achievement name "Paradise Found" is a parody of the 17th Century poem "Paradise Lost" by John Milton. When Sonic achieves a Good Future in any of the zones (via destroying the Metal Sonic projectors and roboticizers), he is essentially "founding" a paradise for the future of that zone.
Contributed by Funland47
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There's an extra special stage that can be accessed through the sound test menu by setting all the 3 selections to 07. According to a former Sega Europe QA Tester, an extra Time Stone was to be collectible here. When collected in addition to the previous Time Stones, "S" TV monitors would appear where the ring TVs were previously located. When broken, Sonic would morph into a lesser version of Super Sonic, which would simply grant invincibility and greater speed. Graphics would not change.
Contributed by Funland47
In the XBLA/PSN version, the achievement for time travelling for the first time is called "88 Miles Per Hour", a reference to Back to the Future.
Contributed by Takahashi2212
The Japanese/European soundtrack for the game is different compared to the American version. An example of this would be the intro and ending songs. In the Japanese version, the intro song is "You Can Do Anything" while the ending song is "Believe In Yourself" by Cosmic Eternity. In the American version, both the intro and ending songs are "Sonic Boom" by Pastiche. Because the American version got a new soundtrack and the intro and ending FMV sequences were being re-edited to fit in time with the respective music, the game was delayed a few months.
Contributed by gamemaster1991
In the instruction manual of the original US release, Amy Rose was referred to as "Princess Sally." This was possibly done as a way to try to connect the game to the then-airing cartoon, but all references to Sally have been removed in re-releases of the game.
Contributed by gamemaster1991
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When Sonic CD was being remade using Christian Whitehead's self-made engine, he wanted to include a new level called "Desert Dazzle". However, SEGA thought it was a bad idea to deviate from the original project. You can actually find a concept image with Christian's initials on it in the sound test by entering PCM:32|DA:8
Contributed by Liraxus
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In the sound test if you enter this combination:

FM no.46, PCM no. 12

A message in Japanese will show up with creepy sonic heads in the background. The message translates to "Fun is infinite with Sega Enterprises. - Madin".

The direct translation for the word at the end is Majin, but the context of the word makes it more likely to be spoken as Madin.
Contributed by lasertaco
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Sonic CD was originally going to be a CD enhanced version of Sonic 2, but after Sonic 2's release, it evolved into its own title. It was also titled "Super Sonic" early in development.
Contributed by gamepopper
Standing still for 3 minutes will cause Sonic to jump off screen, saying "I'm outta here!" and cause an instant game over, regardless of how many lives you may have.
Contributed by RayMonkay
All level titles in Sonic CD are alliterations.
Contributed by RayMonkay
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In Sonic CD, a DJ scene and a Batman tribute image can be brought up in the game sound test area by entering the following codes respectively:

FM NO.42 PCM NO.03 DA NO.01
FM NO.42 PCM NO.04 DA NO.21
Contributed by DidYouKnowGaming