Sparks may fly on Capitol Hill Thursday at a hearing on the subject of media megamergers in which both QVC topper Barry Diller and Tele-Communications Inc. chieftain John Malone will testify.Setting for the event will be Sen. Howard Metzenbaum’s (D-Ohio) Senate antitrust subcommittee. Metzenbaum, a leading cable TV basher in Congress, has also been critical of the recent spate of announcements involving cable/telephone company alliances. He’s expected to use the forum to extract pledges from Malone that the merged Bell Atlantic/TCI venture won’t squelch competition in the two industries. Other witnesses who will testify include Sen. Bob Kerrey (D-Neb.), who has spoken out favorably in support of the Bell Atlantic/TCI deal, and Henry Geller, a D.C. public interest lawyer.