Command to mute and unmute a microphone

Go to System Preferences then Keyboard and click Shortcuts then Custom Shortcuts:

Click on Add

Fill in:

Toggle Microphone

and

amixer set Capture toggle

For USB webcams you need to chose the device (-c 1), or maybe another number.

amixer -c 1 sset Mic toggle

Click Apply and then associate a new key with this command (e.g. the Pause/Break key).

You can mute the microphone with

amixer set Capture nocap

and unmute the microphone with

amixer set Capture cap

I switch between a USB webcam/mic and my internal mic and the other solutions typically work on the "default" device which is often not the device I'm actively using so I wrote this to mute all microphones

pacmd list-sources | \
        grep -oP 'index: \d+' | \
        awk '{ print $2 }' | \
        xargs -I{} pactl set-source-mute {} toggle \
        && pacmd list-sources | \
        grep -oP 'muted: (?:yes|no)'

amixer set Capture toggle && amixer get Capture | grep '\[off\]' && notify-send "MIC switched OFF" || notify-send "MIC switched ON"

Simply mute/unmute with this command:

amixer -D pulse sset Capture toggle

You can also add notification to make sure it's on or off, as per vladimirich answer to the same question

amixer -D pulse sset Capture toggle && amixer get Capture | grep '\[off\]' && notify-send "MIC switched OFF" || notify-send "MIC switched ON"

Inspired by Mark Rooney's answer on muting/unmuting sound.

pacmd is the command line interface to PulseAudio (the sound subsystem used in recent releases). I don't know what the exact command is you'd need but I think you'd want to play with the set-sink-input-mute function.

pacmd is interactive when run without instructions so you have a good opportunity to play around with it and convert that into a one-line function for toggling mute.

the gnome-shell extension nothing to say, which can be installed from its extensions.gnome.org page, provides a microphone icon, mouse and keyboard control, and walkie-talkie style push-to-talk.

On 20.04, I see option to set a shortcut for the same under Settings -> Keyboard Shortcuts -> Sound and Media.

There is no default key assigned though.

To toggle mute of default microphone in pulseaudio:

1. Make sure that you have pacmd (from pulseaudio-utils package) and notify-send (from libnotify-bin).
2. Use this script:

#!/bin/sh

pacmd list-sources | awk '\
BEGIN {default_found=0;}

/^[\t ]*\*/ {default_found=1;}

/^[\t ]*name:/ {
    if (default_found) {
        name=$2;
        gsub("[<>]", "", name);
    }
}

/^[\t ]*muted:/ {
    if (default_found) {
        if ($2=="yes") {
            mute=0;
            icon="microphone-sensitivity-medium";
            status="unmuted"
        } else {
            mute=1;
            icon="microphone-sensitivity-muted";
            status="muted"
        }
        system("pacmd set-source-mute " name " " mute);
        system("notify-send --expire-time 1000 --icon " icon " Microphone: " status);
        exit;
    }
}

/^[\t ]*index:/{if (default_found) exit;}'

You can mute with:

/usr/bin/amixer -q -c 0 sset 'Master',0 mute

Unmute:

/usr/bin/amixer -q -c 0 sset 'Master',0 unmute

You just need to replace 'Master' with the appropriate mixer name, on the terminal use "amixer" to get a list of mixer devices.

About setting the keyboard shortcut check the answers for How can I find which command is bound to a given keyboard shortcut?