The audit of the city utilities department’s water billing procedures is being fast-tracked and expanded, Mayor Kevin Faulconer announced Thursday, as officials seek more answers about how 343 residents were overcharged by as much as $420 due to one employee’s misreading of water meters. Residents began raising questions about their bills with city officials at the start of the year and, disappointed with the response, many went to local media in an effort to get more thorough answers. Some were told by the Public Utilities Department that there was nothing erroneous about their bills.