Why are they using artwork from Sonic Adventure? That's misleading....
Still, CAN YOU FEEL THE SUNSHINE?
That is because the soundtrack was released in Japan in 1998, during Sonic's redesign. What Amazon has is the original album art IIRC.
Also, while we're semi on this topic, fun fact, the Japanese version of Sonic 3D Blast for Saturn was released after Sonic Adventure (late 1999 infact). So, they created a sort of hybrid design between Classic Sonic and Modern Sonic for the art in the manual and cover art.