Found in the following dictionaries
OTCWL2014 (NASPA Word List 2014)
"Unmute" Definition
verb
To restore the sound output (on a speaker or other audio device), having previously muted it.
Source: Wiktionary
To remove a mute from (a musical instrument); to allow (a person who or thing which has previously been silenced) to be heard.
Source: Oxford Dictionary
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