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I'd agree with the Post-Rock suggestion. Something like this would give the game a much more emotional theme to it, I think.

If not that, anything by E.S. Posthumus.

As much as I love metal, I can't really see many metal instrumentals fitting into a WW2 setting.

either something period correct or much more likely something based on the sort of songs used in movie soundtracks for wwii movies. so watch a few of those and see what we as amrericans(or wherever)in this time period looking back tend to think it should sound like because of the media.

Rooster by Alice in Chains is all I need.

Big Band Jazz

Try searching for songs by these artists:

• Audiomachine
• KPM Music
• Ivan Torrent
• Epic Score
• Two Steps From Hell
• E.S Posthumus
  Or just be original and compose some songs, or get someone to compose them for you to avoid copyrights and such...(not sure how this works for games).

some energetic metal / hard-rock.

country

This would certainly not be out of place.

Well, it always depends on the moment of the game. Some country and southern rock would be great.

Carmina Burana.

The Pictures at an Exhibition.

i think a calmer version of dub step or insane version of dub step depending on the levels and some alternative and rock. i love this kind of music in games

I don't know if this is the answer you are looking for but i am strongly convinced that as a developer you should not ask the audience what kind of music they would like to hear in the game you are developing. In stead I think it is far more interesting to work with a music producer with an artistic mindset and trust his skills and capability to emphasize with the game he is working on..
That way you'll get truly creative and attractive outcomes. Wich is something I love about indie games; the use of a creative producer in stead of the idea of sticking to a "genre", think of World of Goo wich had a brilliant soundtrack ( and probably so much more games ) and is not a genre at all for me.

Atmospheric, 21st Century-Classical ala this or this and rock.

Some Vera Lynn, duh. Haven't you listened to The Wall?

Do some research on Richard Wagner... No music is more fitting for parts of WWII than his.

http://www.youtube.com/watch?v=Ns6MBQXtyMk or most of the manowar stuff

smooth jazz

Of course what else beside the music of that era anyway

Thanks to everyone for your help.