Pulseaudio should have hooks
# cat /etc/pulse/default.pa
load-module module-switch-on-connect
load-module module-exec
load-module module-exec arguments="path/to/your/script.sh %s"
(where %s resolves on trigger to the name of the sink added)
Your script.sh should then match the first argument to the name of the sink you want to control, and then run
# path/to/your/script.sh
if [ "$1" = "THESINKIWANT" ]; then
pactl set-sink-volume $1 40%
fi
In this thread: