Great topic :)
My favourite film music is from Alfred Hitchcock's 'The Birds', when the school children are singing and you see crows slowly gathering on a climbng frame in the background.
The images you see make you feel pretty tense but the music is completely childlike & carefree. The childrens voices are source music (i.e. within the diegesis) which I think increases the tension because they are unaware of the danger.
Its a childrens roundelay song, and every time they reach the end of a verse and start the next one it sends shivers down my spine!
http://www.youtube.com/watch?v=FVDT4bWPypk
[youtube]FVDT4bWPypk[/youtube]
