ダウンロードしたffmpegバイナリを呼び出した際にPermission deniedがでて起動することができない。

private IEnumerator ffmpeg( string a_path )
{
 input = "\'"+a_path+"\'";https://qiita.com/drafts/new
 output = "\'"+UnityEngine.Application.persistentDataPath + "/work/"+"hoge.mp4"+"\'";

 string ffmpegExePath = Application.streamingAssetsPath + "/KirinUtil/ffmpeg_mac/ffmpeg";
 Process process = new Process();


 string option = "-i";
 option += " "+input;
 option += " -vf";
 option += " scale=480:270";
 option += " -an";
 option += " -preset veryfast";
 option += " -tune film";
 option += " -crf 23";
 option += " -threads 2";
 option += " "+output;

 string l_cmd = ffmpegExePath + option;

 process.StartInfo.FileName = "/bin/bash";
 process.StartInfo.Arguments = "-c \" " + l_cmd + " \"";
 process.StartInfo.CreateNoWindow = true;
 process.StartInfo.UseShellExecute = false;
 process.StartInfo.RedirectStandardError = true;
 process.StartInfo.RedirectStandardOutput = true;

 process.EnableRaisingEvents = true;
 process.ErrorDataReceived += new DataReceivedEventHandler(ProcessErrorDataReceived);
 process.OutputDataReceived += new DataReceivedEventHandler(ProcessOutputDataReceived);
 process.Exited += new EventHandler(ProcessExited);

 process.Start();
 process.BeginOutputReadLine();
 process.BeginErrorReadLine();
}