What version you are using ?

I tried the command and got


Code:
# /usr/bin/mencoder 1.wmv -o 1.flv -of lavf -oac mp3lame -lameopts abr:br=56 -ovc lavc -lavcopts vcodec=flv:vbitrate=700:mbd=2:mv0:trell:v4mv:cbp:last_pred=3 -srate 22050 -ofps 24
MEncoder 1.0rc1-3.4.6 (C) 2000-2006 MPlayer Team
CPU: Intel(R) Core(TM)2 CPU          6420  @ 2.13GHz (Family: 6, Model: 15, Stepping: 6)
CPUflags: Type: 6 MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled with runtime CPU detection.
success: format: 0  data: 0x0 - 0xf49f0
ASF file format detected.
VIDEO:  [WMV1]  320x240  24bpp  30.000 fps    0.0 kbps ( 0.0 kbyte/s)
[V] filefmt:6  fourcc:0x31564D57  size:320x240  fps:30.00  ftime:=0.0333
==========================================================================
Opening audio decoder: [ffmpeg] FFmpeg/libavcodec audio decoders
AUDIO: 32000 Hz, 2 ch, s16le, 32.0 kbit/3.12% (ratio: 4000->128000)
Selected audio codec: [ffwmav2] afm: ffmpeg (DivX audio v2 (FFmpeg))
==========================================================================
** MUXER_LAVF *****************************************************************
If you wish to use libavformat muxing, you must ensure that your video stream
does not contain B frames (out of order decoding) and specify:
    -lavfopts i_certify_that_my_video_stream_does_not_use_b_frames
on the command line.
REMEMBER: MEncoder's libavformat muxing is presently broken and will generate
INCORRECT files in the presence of B frames. Moreover, due to bugs MPlayer
will play these INCORRECT files as if nothing were wrong!
*******************************************************************************
Cannot initialize muxer.
Exiting...
#

I am using MEncoder 1.0rc1-3.4.6