Anybody remember Vin Garbutt? Now there's a master mumbler...fun to listen to, though.
And KB, I think there's a generational/developmental thing going on...seems like monotone mumbling has always been considered "cool" among adolescents. Remember James Dean? I agree that reading aloud (with encouragement and emphasis on drama and meaning) would be a great way to go.
My pet peeve in popular speech these days is "like". As far as I can tell, this started with the beatniks in the 50s and was resurrected by the hippies in the late 60s, along with lots of other "beat" phrases. But it seems to have taken over the English-speaking world, a kind of verbal kudzu.