This has turned out to be far more difficult to find out than I would have guessed. I still don't know! They also sell an optional "voice controller", I would assume that uses the same tech. Maybe we can figure it out through those specs.

I think we can safely assume it is bluetooth (as the streaming stick is usually behind the TV, out of line-of-sight.

Plus they need to use bluetooth for the voice controller anyway - good luck transmitting voice data over IR :)

the remote for the fire tv is bluetooth[1], so I think that is a safe assumption.

and the picture of voice over IR made me think of the IR stream coming out like morse code...

[1] http://zatznotfunny.com/2014-04/tale-three-remotes/

If it's like the Fire TV remote it's bluetooth and the microphone's inside it.

Could it be using the wifi to transmit the voice commands?

The Roku 3 uses Wi-fi direct: https://en.wikipedia.org/wiki/Wi-Fi_Direct

I wonder if it's that or BT.

Same as regular FireTV, bluetooth