‚ÄčLONDON — Peter Oxendale, a onetime glam rocker ("We all have skeletons," he says), is perhaps the world's leading forensic musicologist, the person musicians call when they believe someone has ripped off their work. In a penthouse overlooking the English Channel, he analyzes songs, everything from pop hits to classical pieces, until he is sure there has been an infringement, or not.So it's a shame he clammed up recently when asked about today's highest-profile copyright question: Did the British pop star Ed Sheeran steal from Marvin Gaye's "Let's Get It On" for his own "Thinking Out Loud"?Asked his opinion of the case, which is now the matter of a lawsuit in New York, Mr. Oxendale pretended to zip his mouth. Did that mean he had a client on either side?"I may or may not be involved in a case featuring a pop star whose name I couldn't possibly mention," he said.