the session.AudioTranscode method is broken, I believe.  Here's the code:

# Converts an audio to another format
            def AudioTranscode(number_of_frames, frame_rate, number_of_channels)
                params = [APIParam.new("srcFilePath", src_file_path ),
                            APIParam.new("destFilePath", dest_file_path),
                            APIParam.new("callbackURL", callback_url),
                            APIParam.new("numberOfFrames", number_of_frames),
                            APIParam.new("frameRate", frame_rate),
                            APIParam.new("numberOfChannels", number_of_channels)]

                Transport.execute_command_post(APICommand.AudioTranscode, params, @account_login)
            end

 Note the first 3 APIParams: src_file_path, dest_file_path, and callback_url - are not params that can be passed into the method.  In the video transcode method, just below this, they are params that can be input.

 Just an fyi, it's an easy fix to add the params, I've done it and it seems to work.