Some audio tracks are too quiet to be heard comfortably without amplification. This becomes a problem when your audio equipment cannot amplify the signal for you. The -softvol option directs MPlayer to use an internal mixer. You can then use the volume adjustment keys (by default 9 and 0) to reach much higher volume levels. Note that this does not bypass your sound card's mixer; MPlayer only amplifies the signal before sending it to your sound card. The following example is a good start:
mplayer quiet-file
-softvol -softvol-max 300
The -softvol option works by controlling the volume audio filter. If you want to play a file at a certain volume from the beginning you can specify volume manually:
mplayer quiet-file
-af volume=10