Here is an example index.theme file:

[Sound Theme]
Name=Birch
Name[fr]=Bouleau
Comment=Sound theme using wooden instruments
Comment[fr]=Theme utilisant des instruments en bois
Inherits=wood,default
Directories=stereo 5.1

[stereo]
OutputProfile=stereo

[5.1]
OutputProfile=5.1

The corresponding directory tree in the /usr/share/sounds directory could look like this:

birch/index.theme
birch/stereo/evolution-urgent-message.oga
birch/stereo/evolution-urgent-message.wav
birch/stereo/fr/evolution-urgent-message.oga
birch/stereo/evolution-urgent-message.sound
birch/5.1/evolution-urgent-message.oga

Where birch/stereo/evolution-urgent-message.sound contains:

[Sound Data]
DisplayName=Evolution urgent message
DisplayName[fr]=Message urgent dans Evolution

In this example a lookup of "evolution-urgent-message", with a 5.1 system preference and no localization, would get the "birch/5.1/evolution-urgent-message.oga" sound file.