hehe, that is exactly the same question a friend of mine wanted me to answer yesterday in a mailinglist we are in. She thought I'd would know (I'll copy and paste this):
"Hahaha, I'm not an expert myself, actually. But I'll give it a try.
The reason you cannot describe the sound of a Viking Metal band is that Viking Metal does not have a particular sound
(simple as that)
It's more the band's lyrics and and their believes that make them
"Viking": Norse mythology, pagan believes, nature and so on (apparently also Vikings are a common theme
). Viking Metal is often seen as a sub-genre of Black Metal and it is also often influenced by Folk music - but this is not a must!
When you think about it, it's the same with White Metal which can also be found in all forms of Metal. It's only the subject of the lyrics that makes them White.
And to name a few Viking Metal bands: Amon Amarth, Menhir, Mithotyn, Thyrfing, Einherjer, Forlorn, Windir. These are all that come to my mind right now, I'm sure there are tons of others. Oh yes, forgot about Enslaved. I think it was them who came up with the term Viking Metal in the first place."
So, and now you can all laugh at me and point at my errors in the explanation. I'm sure Ziu will have to add something