When you manage to say all that in the subject, it means it's too long.

But the problem is that we use XDG_DATA_DIRS. Which one of the directories
should we select? Or should we force using $(datadir)/sounds?

Since you add only /usr/share/sounds to the shortcuts list, not all
$XDG_DATA_DIRS/sound, it would seem logical to open the file chooser in that
directory too.

The first part of XDG_DATA_DIRS perhaps? 

I guess it would even be OK to hard code that to $(datadir)/sounds since if XDG_DATA_DIRS is not set (wich it usually is not) it would default to that dir anyway (plus /usr/share/local)

The default for g_get_system_data_dirs() is:
- when XDG_DATA_DIRS isn't set:
-> /usr/local/share/:/usr/share/
- when set, well, all bets are off.

I'd hard-code it to $(datadir)/sounds myself...

Yeah /usr/share/sounds is the only sane default. 
Since that is the only place where a newly installed system will have sounds...

Created attachment 319874 [details]
Set a default directory

We use whatever we managed to insert first as a shortcut folder for the default directory. Not tested.

Fixed in 2.24.0.1-6