A little script to control volume for the active window

set_active_window_volume.sh

#!/usr/bin/bash
# simple script to set the volume of pulseaudio sinks for the currently focused window
# takes a single argument which is passed to pactl to set the volume; [+-]n% to adjust by n% up or down, respectively

# from https://superuser.com/a/784102/319302
pidtree() (
    [ -n "$ZSH_VERSION"  ] && setopt shwordsplit
    declare -A CHILDS
    while read P PP;do
        CHILDS[$PP]+=" $P"
    done < <(ps -e -o pid= -o ppid=)

    walk() {
        echo $1
        for i in ${CHILDS[$1]};do
            walk $i
        done
    }

    for i in "$@";do
        walk $i
    done
)

# get pid of currently focused window
focused_pid="$(xdo pid)"

# get a list of pids of all related (parent, child) process
pidlist="$(pidtree "$focused_pid" | head -c -1 | tr '\n' ':')"

# find pulseaudio sink ids corresponding to the pids we found
sink_ids=($(pactl list sink-inputs | awk -v pidlist="$pidlist" \
'BEGIN {
    # transform pids into an array (pids) and put the pids in the key so we can use "in"
    split(pidlist,pidarr,":")
    for(i in pidarr) pids["\""pidarr[i]"\""]=""
}
/^Sink Input/{n=split($NF,a,"#"); id=a[n]}
$1 == "application.process.id" && ($3 in pids) {print id}
'))

# adjust the volume of every sink we found
[[ -n "$sink_ids" ]] && for sink_id in ${sink_ids[@]}; do
    pactl set-sink-input-volume "$sink_id" "$1"
done