Voice compression may mean different things:
Speech encoding refers to compression for transmission or storage, possibly to an unintelligible state, with decompression used prior to playback.
Time-compressed speech refers to voice compression for immediate playback, without any decompression (so that the final speech sounds faster to the listener).
Audio level compression refers to a sound recording effect which increases the perceived volume of a sound.
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process of reducing the number of bits per second required to reproduce a voice channel, thereby reducing the transmission bandwidth for that channel.