[FFmpeg-user] convert video from mkv to mp4 failed

[mz]

Hey, Guys:
 I'd just convert my videos from mkv to mp4. The source has several substreams as bellow: 


---------------------------------------------------------------------------------------
  Metadata:
    creation_time   : 2013-03-27 14:06:48
  Duration: 00:44:21.98, start: 0.000000, bitrate: 4884 kb/s
    Stream #0:0(chi): Video: h264 (High), yuv420p(tv, bt709), 1280x720 [SAR 1:1
DAR 16:9], 25 fps, 25 tbr, 1k tbn, 50 tbc (default)
    Stream #0:1(chi): Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
    Metadata:
      title           : 鍥借
    Stream #0:2(chi): Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s (default)
    Metadata:
      title           : 绮よ
    Stream #0:3(chi): Subtitle: subrip (default)
    Metadata:
      title           : 绠€浣
    Stream #0:4(chi): Subtitle: subrip
    Metadata:
      title           : 绻佷綋
    Stream #0:5(eng): Subtitle: dvd_subtitle, 720x576
    Metadata:
      title           : 鑻辫

------------------------------------------------------------------------------------------
First, pick out the subtitle:
ffmpeg -i  HDJ.Sergeant.Tabloid.Ep01.20130311.HDTV.720p.
x264.2Audio-CHDTV.mkv -map 0:3 a.srt


Then, I use the following command to do converting works: 
ffmpeg -i HDJ.Sergeant.Tabloid.Ep01.20130311.HDTV.720p.
x264.2Audio-CHDTV.mkv -map 0:0 -map 0:1 -map 0:2 -c:a copy -c:v -vf subtitles=a.
srt HDJ.Sergeant.Tabloid.Ep01.20130311.HDTV.720p.x264.2Audio-CHDTV.mp4



Now, the problem is: the result has no sound and subtile.


Anybody tell me how can I do this!!!!!!!???????