Former Washington, D.C., mayor Marion Barry, who is appealing his cocaine conviction, apparently is going to try to make a living by giving speeches. He's signed on with a speakers agency and will be available to talk about black politics, city government and alcohol and drug addiction. A Barry associate says the former mayor hopes to bring in about $10,000 dollars a month from speaking engagements and also will seek consulting jobs advising corporations on affirmative action matters. Barry, who had to drop efforts to join the faculty of the University of the District of Columbia because of public and campus opposition, says he also plans to spend about 10 hours a week counseling people with substance abuse problems.