1. A computer-implemented method comprising: receiving, at a computer system, an audio signal; determining, by the computer system, an energy-independent component of a portion of the audio signal associated with a spectral shape of the portion; determining, by the computer system, an energy-dependent component of the portion associated with a gain level of the portion; associating, by the computer system, a weight with each Gaussian distribution in a Gaussian mixture model based on a confidence value for estimates that make up the corresponding Gaussian distribution, wherein a speech model or a noise model comprises the Gaussian mixture model; comparing the energy-independent and energy-dependent components to the speech model; comparing the energy-independent and energy-dependent components to the noise model; and outputting, by the computer system, an indication whether the portion of the audio signal more closely corresponds to the speech model or to the noise model based on the comparisons.