You can look at the -af
(audio filter) section of mplayers manual.
mplayer -af sinesuppress, equalizer, comp, pan, resample
Are a few that may help.
Remove a sine at the specified frequency. Useful to
the 50/60Hz noise on low quality audio equipment.
only works on mono input.
The frequency of the sine which should be r
Hz) (default: 50)
Controls the adaptivity (a larger value wi
filter adapt to amplitude and phase changes
smaller value will make the adaptation slower
0.0001). Reasonable values are around 0.001.
mplayer -af equalizer=11:11:10:5:0:-12:0:5:12:12 media.avi
Would amplify the sound in the upper and lower frequency
region while canceling it almost completely around 1kHz.
You can use filters with ffmpeg
too if you want to capture the mics input with it.