/usr/share/sounds/freedesktop/index.theme is in sound-theme-freedesktop 0.8-2ubuntu1.
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 | [Sound Theme]
Name=Default
Directories=stereo
[stereo]
OutputProfile=stereo
|