Mute system sounds when lock the screen on Debian GNOME.

mute-when-lock.service

[Unit]
Description=Mute When Lock
After=default.target

[Service]
ExecStart=/usr/local/bin/mute-when-lock.sh
Restart=on-failure

[Install]
WantedBy=default.target

mute-when-lock.sh

#!/bin/bash

# Mute;
mute() {
    amixer set Master mute > /dev/null 2>&1
    echo "Screen Locked, Sound Muted."
}

# Unmute;
unmute() {
    amixer set Master unmute > /dev/null 2>&1
    echo "Screen Unlocked, Sound Unmuted."
}

# Listen for the screen lock/unlock signal;
dbus-monitor --session "type='signal',interface='org.gnome.ScreenSaver',member='ActiveChanged'" | \
while read -r line; do
    if echo "$line" | grep -q "boolean true"; then
        mute
    elif echo "$line" | grep -q "boolean false"; then
        unmute
    fi
done