/usr/share/mate/autostart/indicator-sound.desktop is in ubuntu-mate-default-settings 18.04.17.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | [Desktop Entry]
Type=Application
Name=Indicator Sound
Exec=/usr/lib/x86_64-linux-gnu/indicator-sound/indicator-sound-service
StartupNotify=false
Terminal=false
OnlyShowIn=MATE
X-GNOME-Autostart-enabled=true
X-MATE-Autostart-enabled=true
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