12. The computer-implemented method of claim 8 , wherein generating the first score comprises using a hidden Markov model of audio data that corresponds to the keyword, and wherein generating the second score comprises using a hidden Markov model of audio data that does not correspond to the keyword.
8. A computer-implemented method comprising: under control of one or more computing devices configured with specific computer-executable instructions, generating a first score based at least partly on a likelihood that a frame, of a window of sequential frames of audio data, comprises audio data corresponding to a keyword, wherein the window comprises the frame and an equal quantity of (1) frames before the frame and (2) frames after the frame; generating a second score based at least partly on a likelihood that the frame comprises audio data corresponding to background audio; determining a difference between the first score and the second score; and determining that the frame corresponds to an end of the keyword based at least partly on the difference being greater than differences determined for the frames before the frame, and differences determined for the frames after the frame.