Before SEGA went third party and halted the production of the Dreamcast, a sequel to Ecco the Dolphin: Defender of the Future was in development. Titled Ecco II: Sentinels of the Universe, the project was never able to be completed, although one website has miraculously found a prototype of the game and shared it for all to enjoy.
The prototype does not contain any music and also has an active debugger enabled. The website includes a version that works on emulators as well as a self-boot CDI that can be burned and played directly on your Dreamcast. You can click here to check it out.
Via Hidden Palace