I was coming in to suggest Gary Jules' version of Mad World, but I see I've been beaten to the punch half a dozen times. (I should never have shown you guys "Kiwi!"
)
Alternately, there are two or three songs by Alice in Chains that might fit the bill
"Nutshell"
"Don't Follow" (you'll have to cut out the last verse, as it becomes upbeat)
"Whale and Wasp" (an instrumental)
It has an odd feel, but Imogen Heap's version of "Hide and Seek" might be useful
If you can find an instrumental version of Pink Floyd's "Is there anybody out there" it would work well.
Their song "Outside the Wall" is also appropriate. Perhaps even "Wish You Were Here" (the song, from the album of the same name)
Nickelback performs the song "Far Away" which is heart-wrenching in the proper context.
A Perfect Circle's "3 Libras" might work in small bits. Particularly an instrumental studio version.
R.E.M.'s "Everybody Hurts" would be great, if (once again) you can find the instrumental. (See a pattern here?)
...
Let me think a while. I know many more than these.
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EDIT: Ooh! I just remembered a song from Pink Floyd that only appeared in their film "The Wall" in three parts, called "When the Ti(y)gers Broke Free". It fits the period you're making this presentation about, perfectly. Unfortunately it's written from the perspective of a child whose father was lost in WW2, but it still may be useful. It seems that there is a version of this song performed by the London Philharmonic Orchestra that would probably be your best bet.