Embodiments may be implemented by a computing device, such as a head-mountable display, in order to use a single guard phrase to enable different voice commands in different interface modes. An example device includes an audio sensor and a computing system configured to analyze audio data captured by the audio sensor to detect speech that includes a predefined guard phrase, and to operate in a plurality of different interface modes comprising at least a first and a second interface mode. During operation in the first interface mode, the computing system may initially disable one or more first-mode speech commands, and respond to detection of the guard phrase by enabling the one or more first-mode speech commands. During operation in the second interface mode, the computing system may initially disable a second-mode speech command, and to respond to the guard phrase by enabling the second-mode speech command.