[MEncoder-users] mplayer -vc null -vo null -ao pcm:fast: Your system is too SLOW to play this!

[Strong]

I dump sound out of avi with this (is it more efficient with mencoder?):

$ mplayer -vc null -vo null -ao pcm:fast 2006.avi

And get this:
MPlayer 1.0rc1-rpm.livna.org-4.1.1 (C) 2000-2006 MPlayer
Team CPU: Intel(R) Celeron(R) CPU 2.26GHz (Family: 15, Model: 3,
Stepping: 3) CPUflags:  MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled with runtime CPU detection.
mplayer: could not connect to socket
mplayer: No such file or directory
Failed to open LIRC support. You will not be able to use your remote
control.

Playing 2006.avi.
AVI file format detected.
VIDEO:  [FMP4]  352x244  24bpp  25.000 fps  709.0 kbps (86.6 kbyte/s)
Clip info:
 Software: MEncoder 1.0rc1-4.1.1
==========================================================================
Forced video codec: null
Opening video decoder: [null] Null video decoder
VDec: vo config request - 352 x 244 (preferred colorspace: BGR 24-bit)
VDec: using Planar YV12 as output csp (no 0)
Movie-Aspect is 1.57:1 - prescaling to correct movie aspect.
VO: [null] 352x244 => 384x244 Planar YV12
Selected video codec: [null] vfm: null (NULL codec (no decoding!))
==========================================================================
==========================================================================
Opening audio decoder: [liba52] AC3 decoding with liba52
Using SSE optimized IMDCT transform
Using MMX optimized resampler
AUDIO: 48000 Hz, 2 ch, s16le, 256.0 kbit/16.67% (ratio: 32000->192000)
Selected audio codec: [a52] afm: liba52 (AC3-liba52)
==========================================================================
[AO PCM] File: audiodump.wav (WAVE)
PCM: Samplerate: 48000Hz Channels: Stereo Format s16le
[AO PCM] Info: Faster dumping is achieved with -vc null -vo null -ao
pcm:fast [AO PCM] Info: To write WAVE files use -ao pcm:waveheader
(default). AO: [pcm] 48000Hz 2ch s16le (2 bytes per sample)
Starting playback...
A:   2.7 V:   2.4 A-V:  0.291 ct: -0.000  62/ 62  0%  0%  0.5% 55 0

           **** Your system is too SLOW to play this!  ****

Why is for my Celeron 2,26GHz and 512Mb RAM?!
And it is at the first.

At the second it does not save all the sound it length is 2 hrs. Ho to
fix this?

And yet another question, Is it possible to:
1. spare just one *specified* channel and save in wave;
2. spare just one *specified* channel and save in mp3 at the specified
rate or even specify variable rate?