je pense que c'est ça

 -af <plugin1[=options],plugin2,...>
 Activate a comma separated list of audio filters and their options.
 Available filters are:
 resample[=srate[:sloppy][:type]]
 Changes the sample rate of the audio stream to an integer srate (Hz). It only supports the 16 bit little endian format.
 channels[=nch]
 Change the number of channels to nch output channels. If the number of output channels is bigger than the number of input channels empty
 channels are inserted (except mixing from mono to stereo, then the mono channel is repeated in both of the output channels). If the number
 of output channels is smaller than the number of input channels the exceeding channels are truncated.
 format[=bps,f]
 Select the format f and bytes per sample bps used for output from the filter layer. The option bps is an integer and denotes Bytes per
 sample. The format f is a string containing a concatenated mix of:
 alaw, mulaw or imaadpcm
 float or int
 unsigned or signed
 le or be (little or big endian)
 volume[=v:sc]
 Select the output volume level. This filter is not reentrant and can therefore only be enabled once for every audio stream.
 v: desired gain in dB for all channels in the stream. The gain can be set from -200dB to +40dB (where -200dB mutes the sound completely
 and +40dB equals a gain of 1000).
 sc: enable soft clipping.
 pan[=n:l01:l02:..l10:l11:l12:...ln0:ln1:ln2:...]
 Mixes channels arbitrarily, see DOCS/HTML/en/devices.html#audio-dev for details.
 n: number of output channels (1-6).
 lij: how much of input channel j is mixed into output channel i.
 sub[=fc:ch]
 Add sub-woofer channel.
 fc: Cutoff frequency for low-pass filter (20Hz to 300Hz) default is 60Hz.
 ch: channel number for the sub-channel.
 surround[=d]
 Decoder for matrix encoded surround sound, works on many 2 channel files.
 d: delay time in ms for the rear speakers (0ms to 1000ms) default is 15ms.
 delay[=ch1:ch2:...]
 Delays the sound output. Specify the delay separately for each channel in milliseconds (floating point number between 0 and 1000).
 export[=mmapped_file[:nsamples]]
 Exports the incoming signal to other processes using memory mapping (mmap()).
 mmapped_file: File to map data to (default: ~/.mplayer/mplayer-af_export).
 nsamples: number of samples per channel (default: 512)