1. A method of selecting a microphone from two or more microphones for a multi-microphone speech processing system operating in a noisy environment, each of the microphones being associated with a respective channel and being suitable for picking up a noisy sound signal having a useful speech component from a main sound signal source (s(t)) mixed with a diffuse noise component, the method comprising the following steps: digitizing the sound signals picked up simultaneously by the two microphones (N, M); transforming the signals (x n (t), x m (t)) picked up on the two channels in such a manner as to produce a succession of frames in a series of frequency bands; applying an algorithm for calculating a speech-presence index of each channel; selecting one of the two microphones by applying a decision rule to the successive frames of each of the channels, which rule is a function both of a channel selection criterion and of said speech-presence index; and implementing speech processing on the basis of the sound signal picked up by the one selected microphone; which method is characterized in that: said transformation of the signals (x n (t), x m (t)) picked up on the two channels is a short-term Fourier transform; said speech-presence index is a confidence index calculated for each frequency band of each frame; and said selection criterion is calculated in frequency bands on only those frequency bands for which the confidence index is greater than a first given threshold.