Sonic Virtual Adventure is likely a fan-made game or ROM hack designed to provide a virtual reality-like experience within the Sonic universe. The gameplay might use modified camera angles and perspectives to give a different feel to the classic Sonic platforming experience. The core mechanics likely derive from the original Sonic the Hedgehog games by Sega.  This demonstrates the creativity of fan developers in experimenting with new game mechanics and presentation.

Sonic Virtual Adventure

Sonic Unlimited v3.5