I think Midwestern is sort of the neutral TV and radio-announcer dialect, so it's really more widespread than just the Midwest.

Standard American Stage English is closest to urban mid-Atlantic, regionwise. (And supposedly also Idaho, which I've heard nicknamed "the pure-vowel-state" by musical directors.)

the fact that so few actors enunciate well enough for me to understand what they're saying

This particular foreigner has no real problem understanding 98% of what actors say. How much are you not getting?

I had a question about that, too. Are you speaking about American actors?

I never understand what Benecio del Toro is saying.

eta: He may not qualify as an American actor?

cannot control their speech well enough to drop an inappropriate accent

this. Although the example I was going to give was Cary Elwes in Poison Ivy so perhaps I am asking too much of that particular film. Still I wonder why they didn't just make the character british.

I never understand what Benecio del Toro is saying.

Have you just been watching him in Usual Suspects? Because that's what he was supposed to do there.

He's often a bit of a mumblemouth, though.

Benicio Del Toro is from, like, Pennsylvania. His mushmouth is pure personality.

I don't actually sit there completely lost as to what's going on, but I mishear words on TV all the time -- a T that might be a D, or G/K, or a couple other consonant switches -- that can change meanings. I get jerked out of the story, when I find myself relying on context to understand meaning rather than relying on hearing the word right the first time. I find most Texas accents mildly mushy, e.g., and the vowel-confusion of anybody from Chicago will usually get me asking, "Wait, what?"

Have you just been watching him in Usual Suspects?

That was the first movie I thought of, but I also had some issues with Sin City.

And Fear and Loathing in Las Vegas