OmniROM developer is modding the Moto X' touchless voice control into the Nexus 5
Not unlike similar cases in which available technology was found laying dormant inside the silicon, the voice processor can be activated by tinkering with software. The voice recognition's availability was never a secret, however. It was revealed in a Qualcomm promotional video.
A proof-of-concept video indicates that bringing the Moto X's touchless controls to Google's phone is really possible. Alas, the mod's current condition is unstable and the Nexus 5 ROM will require a fair bit of nipping and tucking before your Nexus becomes obedient.
via Android Authority