How Dota 2 picks your microphone
Dota 2 voice chat uses your computer's default recording device, and on some setups it points you to Steam's voice settings instead. Making the Voices AI microphone your default input covers both. If Dota 2 still hears your old mic, restart the game after you change the default.
Steam Settings > Voice has a microphone test, so you can hear the AI voice before you queue.

