First, you can run acpi_listen to get the name of the key that is pressed.

acpid configuration

#

Directory

#

/etc/acpid/events

Files

#

Volume Down

#

event=button/volumedown
action=sudo -u karmst XDG_RUNTIME_DIR=/run/user/1000 /home/karmst/scripts/vol_down.sh

Volume Up

#

event=button/volumeup
action=sudo -u karmst XDG_RUNTIME_DIR=/run/user/1000 /home/karmst/scripts/vol_up.sh

Volume Mute

#

event=button/mute
action=sudo -u karmst XDG_RUNTIME_DIR=/run/user/1000 pamixer --toggle-mute