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:
quiet-file -softvol -softvol-max 300
The -softvol-max option specifies the maximum allowable output volume as a percentage of the original volume. For example, -softvol-max 200 would allow the volume to be adjusted up to twice its original level. It is safe to specify a large value with -softvol-max; the higher volume will not be used until you use the volume adjustment keys. The only disadvantage of a large value is that, since MPlayer adjusts volume by a percentage of the maximum, you will not have as precise control when using the volume adjustment keys. Use a lower value with -softvol-max and/or specify -volstep 1 if you need higher precision.
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:
quiet-file -af volume=10
This will play the file with a ten decibel gain. Be careful when using the volume filter - you could easily hurt your ears if you use too high a value. Start low and work your way up gradually until you get a feel for how much adjustment is required. Also, if you specify excessively high values, volume may need to clip the signal to avoid sending your sound card data that is outside the allowable range; this will result in distorted audio.