1. A system for voice activity detection (VAD) within a digitally encoded bitstream, the system comprising: a parameter extraction module implemented using one or more hardware processors and configured to extract parameters from a sequence of coded frames from a digitally encoded bitstream containing speech, the parameters extracted being parameters of a codec used in encoding the sequence of coded frames; a VAD classifier selection module configured to: determine a bit rate of the digitally encoded bitstream; and select a given VAD classifier from among a plurality of VAD classifiers based on the determined bit rate, the given VAD classifier having been trained for the determined bit rate of the digitally encoded bitstream with a training file corresponding to the determined bit rate; and the given VAD classifier implemented using the one or more hardware processors and configured to operate exclusively in a bitstream domain with input of the digitally encoded bitstream to output a VAD decision indicative of whether or not speech is present in one or more of the coded frames, the VAD decision determined through evaluation of the one or more of the coded frames based on bitstream coding parameter classification features and the parameters extracted.