Odd. Are you in the correct screen?
Another way to get there:
Pull down the notification shade, hit the gear icon to get into settings.
Scroll down and select Google.
Scroll down and select Search.
Scroll down and select Voice.
Once there I see Languages, Ok Google detection, Hands-free, Speech output, Offline speech recognition, Block offensive words and Bluetooth headset as my options.
Tapping Hands Free gives me two options - allow requests with device locked for Bluetooth and for Wired Headsets.