I finally managed to make this work using udev. So if someone wants the same behavior here are the steps:

First we need to create a file /etc/udev/rules.d/hdmi_sound.rules with the following content:

    SUBSYSTEM=="drm", ACTION=="change", RUN+="/usr/local/bin/hdmi_sound_toggle"

this will make udev execute the script hdmi_sound_toggle every time there is a change in HDMI connection. That script must have execution permission and the contents are as follows:

#!/usr/bin/env python



import subprocess

from syslog import syslog



def output(cmd):

    return subprocess.check_output(cmd, shell=True)



# the following variables may need some modification.

user = "my_username"

card = "/sys/class/drm/card0"

dev_speaker = "output:analog-stereo+input:analog-stereo"

dev_hdmi = "output:hdmi-stereo+input:analog-stereo"

#



interfaces = output("ls {0}".format(card), ).split("

")



vga = filter(lambda x: "VGA" in x, interfaces)[0]

hdmi = filter(lambda x: "HDMI" in x, interfaces)[0]



syslog("HDMI connection was changed!")



hdmi_connected = output("cat {0}/{1}/status".format(card,hdmi)).startswith("connected")

title = "HDMI was {0}".format("connected" if hdmi_connected else "disconnected")

message = "Audio output has changed to {opt}.".format(opt = "HDMI" if hdmi_connected else "built-in speakers")



cmd = "sudo -u " + user + " /usr/bin/pactl set-card-profile 0 " + (dev_hdmi if hdmi_connected else dev_speaker)



syslog("HDMI was connected." if hdmi_connected else "HDMI was disconnected.")

try:

    a = output(cmd)

    output("sudo -u {0} notify-send "{1}" "{2}"".format(user, title, message))

    syslog("Audio output changed.")

except Exception as ex:

    syslog("Error changing output device: " + str(ex))

Probably this can be easily made in bash, but as my main language is python I used it. Everything works except the notification: it doesn't show up, I really don't know why. If someone knows how to fix it please say something.

Note: the names of script/udev rule can be changed, but you need to use the full path.