/etc/acpi/always-mute.sh is in acpi-support 0.142-6.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | #!/bin/sh
# on ThinkPads, the volume keys always performs mute,
# regardless of the previous state; for the moment
# we fake this by "always" unmuting and then remuting.
# Kludge/behaviour copied over from 'thinkpad-keys'
# -Paul Sladen 2007-09-13
[ -f /usr/share/acpi-support/key-constants ] || exit 0
. /usr/share/acpi-support/key-constants
#acpi_fakekey $KEY_VOLUMEUP
acpi_fakekey $KEY_MUTE
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