A report released Tuesday by the Metropolitan Transit Authority (MTA) shows Bronx and city subways are getting cleaner.According to the MTA?s passenger environment survey, more than half the subway cars are considered clean. The survey, which was conducted in 56 areas across the Bronx and city, also showed that both buses and subways improved in the graffiti category.The report also found that announcements on subway platforms are nearly impossible to hear. Only about a quarter of announcements made during the first half of this year were understandable.