WASHINGTON, July 11 — The Transportation Security Administration spent $6.9 million to outfit the federal air marshal corps with voice-capable wireless handheld computers for in-flight use and then subsequently banned any voice usage because of potential interference with cockpit functions, according to interviews with several air marshals. THE HANDHELD DEVICES, made by Palm Inc. and commonly known as “personal digital assistants” (PDAs), have become a running joke, in part because they are unwieldy to use, according to interviews more than a dozen air marshals and administrators within the air marshal program due to the unwieldy nature of their use. Although...