NEW YORK (AP) -- It's been a rough summer at the box office, and movie theater chains are feeling the pain. It got worse last week when AMC Entertainment said U.S. box office grosses are sinking. Paul Dergarabedian, a senior media analyst for comScore who has followed the industry for more than 20 years, says movie theaters are always vulnerable if people aren't excited about what's playing, but things can change quickly....