Most modern tellies have a tone setup somewhere in the menu system - check out the manual, or post the exact model number on here.
My cheapo 19" has a 5 band equaliser, so I boost the upper mid and the top frequencies to make speech more intelligible.
Having said that, there does seem to be a craze in film and TV at the moment for all people to mumble and/or speak with their mouths full of food - I think the directors are under the impression that it lends authenticity (or maybe all Americans really do only speak when their gobs are full?)