MPlayer has two options for increasing volume above the sound card's maximum setting. Adjusting MPlayer's software volume, or using an audio filter to amplify the audio. The Audio filter method tends to work better.
Software Volume or softvol adjusts you volume using software amplification, not your sound cards hardware volume. To use it add
-softvol -softvol-max <10.0-10000.0>. To set the maximum amplification level in percent (default: 110). A value of 200 will allow you to adjust the volume up to a maximum of double the current level. With values below 100 the initial volume (which is 100%) will be above the maximum, which e.g. the OSD cannot display correctly. For example,
mplayer -softvol -softvol-max 200 media.avi
The other method is to turn on an audio filter to increase the volume. To use it add
-af volume=<v>:<sc>. See HowTo Wiki for more details.
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