This is pretty vague but in any film when someones gois to the deep south (or any swamp) there is this slow guitar music, it sound sort of depressing but cool, is this Blues or blugrass (or something else), and is there a good cd with this stuff, maybe mostly instramental, maybe a broad compilation TIA.
I think you may be referring to a style of playing known as 'slide guitar', which is associated with blues and the deep south . Naz and Andy Hughes will be along soon and recommend some suitable CD's!!
The music usually played in such films as Southern Comfort and The Big Easy is swamp blues (often performed by Ry Cooder); the faster, fiddle-and-banjo variety is Cajun (or Zydeco if played by those of African-American rather than French Canadian descent). Bluegrass is more mid-Western.
Additionally, there's very little if any Bluegrass that doesn't include vocals. It's really a means of telling a story set to music. The genre has a strong Scots/Irish background from the original settlers of the eastern seaboard of the U.S. and it's origin is in the mountain country of Tennessee, West Virginia and Kentucky, where the term "Bluegrass" orignates.