Allow requests with device locked
For Bluetooth devices
For wired headsets
in Settings for Google Voice search.
However, none of my devices ( Sky Vega racer3 and Samsung Galaxy Tab 10.1) seems to be responsive to "Okay Google" using either a wired headset or my bluetooth headset.
Both of my devices respond correcctly to "Okay Google" with or without any bluetooth devices or wired headsets when the device is not locked.
None of my devices has a lock screen.
Ultimately, what I want to do is to be able to send Hangouts messages or ask for the time while I'm riding my bicycle without having to pull my cellphone out of the pocked and turn on the screen.