Abort all UI, scout update, vehicle handling, and weapon balance updates, this needs fixing immediately. I am dead serious.
You can find this loading music in this directory. \empires\sound\common\music\loading I assume it's for loading considering the folder name.
Oh, well that's disappointing. It says custom loading panel in the svn, and I see crossroad screenshots added at the same time so I would think it wasn't exactly tied to the html stuff, but it was added 2013 so yeah I guess I'm wrong. Now I'm extra sad.
New bug: it seems to ignore the sound volume setting, so it cannot be turned off except by removing the files it uses.
I believe that's because the loading screen menus use FMOD instead of the standard Source sound system.