Daniel Barenboim, who was scheduled to open the Washington Opera season Oct. 26 conducting "Don Giovanni," is in Paris suffering from an inner ear infection. John Mauceri, who was once music director of the Washington Opera, is adjusting his Covent Garden schedule to substitute for Barenboim for the six performances here. "Don Giovanni" will be the third Mozart opera produced jointly by the Washington Opera and L'Orchestre de Paris, where Barenboim is music director. He has previously conducted "Don Giovanni" to critical acclaim at the New York City Opera . . .

As someone who has a first-hand understanding of the need for handgun control, Sarah Brady, the wife of White House press secretary James Brady, who was severely wounded in the attempt on President Reagan's life, has become a board member of Handgun Control Inc. Brady said she decided to become involved in handgun control after the Senate's recent vote to weaken the 1968 Gun Control Act. She will be testifying on the issue at a House Judiciary Committee Hearing in New York City Oct. 28 . . .

Media morsels: Former Washington Star movie critic Tom Dowling has been named book critic of the San Francisco Examiner . . . New York Times chief Washington correspondent Hedrick Smith has taken a one-year leave of absence to become a visiting journalist at the American Enterprise Institute, where he will be working on a book on power in Washington . . . Washington Post columnist Hobart Rowen is to receive the 1985 Annual Private Sector Award of the Washington International Trade Association, in recognition of those who have made significant contributions in the field of international trade. The public sector award of the association goes to Commerce Secretary Malcolm Baldrige . . .

Nicholas, Glover and Wray, Barbara Cook's backup group at Ford's Theatre, evoke warm memories of sisters' singing groups. One of the group, Sheilah Glover, is the daughter of Washingtonians Charles and Virginia Glover and attended Cathedral School. Yes, she is of the Glover as in Glover-Archbold Park family . . .