The MBX Expansion System was an add-on developed by Milton Bradley for the TI-99/4A which included a new, pistol-grip, multi-button joystick, a 64-key membrane keypad, a voice synthesizer/voice recognition system and a microphone. The system also had its own speakers.
Its capabilities could only be used by specially written software. Existing TI-99 software wouldn't be able to use the features of the MBX Expansion System.
Some software could use either the TI Speech Synthesizer or the MBX voice syhthesizer, dependin on the hardware found.
Milton Bradley initially planned to release its own console, then decided that the console market was too crowded, but insread of discarding all developement work repurposed it as an expansion system for the TI-99/4A.