How can I stop a belt brake (band brake) from squealing?
That style of drum brake relies on friction between steel and leather. If it is setup to stop reasonably well, they will always make some noise. Quite often, when new, they do indeed howl like Baoine Sidhe. 

This will get better with use. From the photos, it appears your brakes are fairly new? Assuming that they are actual photos of your bike...

Rubbing talcum powder into the leather strap will help quiet the brake, but it may also have an adverse effect on your braking performance. Best to let it work out naturally, with time and use. 

I hope that helps.