When using kivy on a Debian system running pulse audio (gnome) you would encounter issues with the sound volume due to the fact that kivy would set the volume to 100% each time it uses the soundcard.
The problem is in pulse audio, what’s new 🙁 as it’s the from the same author who brings us systemd.
You can remove pulse audio as much of gnome depends on it.
You can however disable pulse and use good old alsa.

Disable pulse in /etc/pulse/client.conf uncomment the line “autospawn = yes” and set it to “autospawn = no”
Install gnome alsa mixer, sudo apt-get install gnome-alsamixer
Restart alsa, sudo alsa force-reload
Or use alsamixer (console) to control the volumes.

Tried to use key shortcuts in gnome but that doesn’t work correctly.
Just use trusted xbindkeysrc always works 🙂

install xbindkeys, sudo apt-get install xbindkeys
Use this .xbindkeysrc file and your done (after logout and login)

#Alsa Volume Down
"amixer set Master 3- unmute"

#Alsa Volume Up
 "amixer set Master 3%+ unmute"

#Alsa mute/unmute toggel
 "amixer sset Master toggle"