求助：mpg转flv后没声音！！

mpg转flv后没声音，查看媒体信息，音频显示 Format : 706D，这是什么鬼？应该是aac啊，软件默认是aac，而且，为什么不能选择mp3呢？为什么强制aac，而且转出来还没声？
完整媒体信息如下：
General
Complete name : J:\ Lesson23.flv
Format : AVI
Format/Info : Audio Video Interleave
File size : 791 MiB
Duration : 42mn 29s
Overall bit rate : 2 603 Kbps
Writing application : MEncoder sherpya-r37853+g6d7f566-5.3.1
Writing library : MPlayer
Video
ID : 0
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L3
Format settings, CABAC : No
Format settings, ReFrames : 1 frame
Codec ID : avc1
Duration : 42mn 29s
Bit rate : 2 400 Kbps
Width : 1 024 pixels
Height : 576 pixels
Display aspect ratio : 16:9
Frame rate : 25.000 fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.163
Stream size : 729 MiB (92%)
Writing library : x264 core 148 r2665 a01e339
Encoding settings : cabac=0 / ref=1 / deblock=1:0:0 / analyse=0x3:0x113 / me=hex / subme=7 / psy=1 / psy_rd=1.00:0.00 / mixed_ref=0 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=1 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=-2 / threads=6 / lookahead_threads=1 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=0 / weightp=2 / keyint=250 / keyint_min=25 / scenecut=40 / intra_refresh=0 / rc_lookahead=40 / rc=abr / mbtree=1 / bitrate=2400 / ratetol=1.0 / qcomp=0.60 / qpmin=0 / qpmax=69 / qpstep=4 / ip_ratio=1.40 / aq=1:1.00
Audio
ID : 1
Format : 706D
Codec ID : 706D
Duration : 42mn 29s
Bit rate : 192 Kbps
Channel(s) : 2 channels
Sampling rate : 44.1 KHz
Stream size : 58.3 MiB (7%)
Interleave, duration : 23 ms (0.58 video frame)
Interleave, preload duration : 68 ms