Unmute is in the online Oxford and Wiktionary. As well as being used in connection with the controls of an audio device or app, it can be used for removing a mute from a musical instrument. It can also be used figuratively, as in this example quoted by the OED:
Women's expanded interests could...lead to efforts to decrease the distance between 'public' and 'private' worlds, or at least in 'unmuting' the values of the 'muted' half of society.
The earliest citation in the OED is from 1811.
I'll add
unmute as a rare word, along with
unmuted and
unmuting.