From Bulbapedia, the community-driven Pokémon encyclopedia.
The Voice Recognition Unit (Japanese: Voice Recognition System), often shortened to VRU (Japanese: VRSユニット VRS unit), is a peripheral for the Nintendo 64 that allows the player to play games using sounds, specifically a voice. The Wii Speak can be considered its successor.
The device came as a pack-in with Hey You, Pikachu!, and was required to play the game. It is plugged into controller port 4 of the Nintendo 64. It included a clip for clipping it to the Nintendo 64 controller.
Other than Hey You, Pikachu!, the device only worked with one other game—the Japan-exclusive Densha de Go! 64. The VRU was not necessary to play this game, so was not included with its purchase.
The VRU is calibrated for a high-pitched child's voice, so adolescents and adults may have difficulty when using the device. VRUs are region-dependent, so a Japanese VRU cannot be used with a US game, and vice versa. No PAL version was released.