In the E3 1999 demo of Sonic Adventure, Knuckles had a very different voice. This was the same voice actor, but they re-recorded nearly all of his lines before the game's release. Surprisingly, voices for his entire story were included, even though the demo only had two stages for Knuckles.
The game's composer, Jun Senoue, initially went through and scrapped 8 different prototype iterations of the game's main theme song before settling on Open Your Heart.
Sonic Adventure had a limited edition release available two months before the release of the SEGA Dreamcast in North America. SEGA signed a deal with the game rental company Hollywood Video that let people rent a SEGA Dreamcast two months before the release of the console with a selection of games, one of those being Sonic Adventure Limited Edition. Although those copies were never meant to make it into the hands of the public, the copies that did make it to the public usually are incomplete or have a Hollywood Video sticker on the top of the disc.
Big the Cat was purposely created to move at a different pace and to break up the fast-paced action seen so far in the game. Fishing was chosen due to the fact that "he is well... a cat.".
The music for Red Mountain is a remix of "Crying World", the theme from Ristar. It also uses samples from Super Cat's "Ghetto Red Hot (Hip Hop Mix)".
During Sky Chase Act 2, Tails pilots the Tornado 2. The life bar, however, still shows the orginal Tornado 1 life bar. What's odd about this is that there is an unused graphic in the game's files that was going to be used for the Tornado 2, but goes entirely unused.
Some objects were removed the game's original release, but were later discovered in the PC version of the game. The objects include an unused button in Lost World, a tumbleweed object in Windy Valley, a trash can object in Speed Highway, and an icicle object in the Icecap stage.
Six images can be found in the game's data that appear to be part of an old character select screen.
An unused Clock item icon can be found in the game's files. It's possible that it may have functioned the same way as "Chaos Control", or was a way of adding time to E-102 Gamma's levels, as they were all on a strict time limit.
There is an unused spring model that can be found in the game's files. It looks very similar to the springs used in Sonic Jam's Sonic World. Its texture appears to be used for the switches in the final game.
Several unused Tikal voice clips were discovered inside the game's files, including her talking about the Leon badnik, an unknown enemy called "Golah", and a clip referencing that Super Sonic was supposed to be usable in standard levels.
A prototype loading screen exists in the game's data. It was used in the Sonic Adventure '98 AutoDemo.
In the first cutscene of Sonic's story, Eggman is seen standing on top of building with a guard rail in front of him. This guard rail was removed, and the roof texture was changed in the DX version of the game.
As far back as 1999, composers in the Sonic series have been sampling from other tunes. The famous "Up and down and all around" and even the "Right!" that can be heard after it in the track Run Through the Speed Highway actually comes from James Brown's Super Bad.
In May 2013, Sonic Retro Researcher Orengefox posted his discovery of a Sonic Adventure prototype that is "without a shadow of a doubt the earliest Sonic Adventure prototype we've acquired". The build contains early Chao Garden music, unused graphics, a nameless Froggy, and several clues about early versions of Windy Valley amongst other things.
In preparation for Sonic Adventure, Sonic Team conducted a trip to Central and South America. This was done because none of the developers had ever seen environments like ruins and jungles, and the team wanted to make a realistic world for the game. Some of the pictures they had taken were used as textures for the game. Tails' Sand Hill sub game was even inspired by people boarding on sand dunes in Ica, Peru.
To celebrate the release of Sonic Adventure, Yuji Naka (former head of Sonic Team) joined a line of fans in Tokyo's Akihabara district waiting to purchase the game. Accompanying him were 4 people dressed as Sonic, Tails, Knuckles, and Amy.
Windy Valley went through major visual changes during the game's development. In the intro sequence, Tails and Big can be seen standing in an early prototype of this stage. In fact, Big isn't even able to access Windy Valley in the final game.
In the first Japanese release for the Dreamcast, a mechanical cowgirl billboard could be seen within Casinopolis. It would moan whenever the player touched it and was removed in international copies of the game for obvious reasons. This version however was also toned down from the Sonic Adventure Auto Demo prototype, where the girl was a fully fleshed playboy-style bunny. [Image credit: evilhamwizard]
The hidden black Chao egg found within the Egg Carrier has a unique texture in the original version for the Dreamcast. It was blue and black with pink spots. This was probably so that it would fit in better with its surroundings. This was changed in later releases of the game to solid black. Opening it will hatch a regular black Chao with seemingly no special qualities, however.
The Dreamcast disk has a PC-readable folder named "extras" that contains nine different wallpapers based on the playable characters, plus one of a Chao. Some of these pictures would later be used in the gallery in Sonic Mega Collection's Extras menu.
In the Dreamcast version of Sonic Adventure, you can explore under the deep water in the Egg Carrier Chao Garden. Under the water lies a secret ring far out of reach. This was removed in the DX update, because the depth of the water was reduced, probably to prevent Chao from swimming out of reach and getting stuck in the terrain.
Knuckles' theme song, Unknown from M.E., incorrectly states that he is a porcupine.
The song "Twinkle Cart", used when Sonic first enters the Twinkle Park stage during his story, is a remade version of Panic Puppet Act 1 from Sonic 3D Blast (from the Genesis version). The composer of both is Jun Senoue, the guitarist and keyboardist who is a key member of the band Crush 40.
The background music in the second area of Windy Valley, The Air, is an arrangement of the music from Green Grove Zone in the Sega Genesis version of Sonic 3D Blast.
Cream has several cameos in Sonic Adventure DX. She flies by after Sonic and Tails clear Casinopolis, she can seen by Big in the same area, and Amy spots her flying by the train station at Station Square.
There are various unused voice clips, all belonging to Tikal. One of these clips are instructions on how to transform into Super Sonic. As such, it was probably a dropped feature.
Sonic Adventure is the first Sonic game to feature downloadable content, as the Dreamcast was the first ever video game console to have online capabilities out of the box. DLC ranged from celebratory holiday decorations in the Adventure Fields and a few levels, such as on Christmas and Halloween, to national ranking contests and the addition of the Chao Black Market.
Jon st. John, the voice of Duke in the Duke Nukem games, also voiced Big the Cat for several Sonic games starting with Sonic Adventure.
An image of some staff members from Sonic Team, and a picture of a blue spined hedgehog, can be found in the game's data.
Dred Foxx, who voices Parappa in Parappa the Rapper, rapped the lyrics for "Unknown from M.E." (Knuckles' Theme).