How to boost playback volume in real time on media recorded with a very low volume

I have never heard a satisfactory answer to this often misunderstood question, let me explain.

Lets say I have a sound card and earphones/speakers that can play back audio loud enough in most cases. This is great but the problem is that you always find people who do not know how to record audio, from Youtube video's to music.

So now you end up with a audio playback that only uses 10% or less of the capacity of your sound hardware, in vista/win 7 you will see this frequently in the mixer with the volume pushed up to max but the green sound level only goes up a millimeter or two.

I am looking for (preferably free) software or a method to boost the sound level of any audio from any source in real time to use more of my hardware capacity similar to what VLC media player can do.

Oh and please, do not tell me it is impossible. I am not trying to boost the volume past what my hardware is capable of, I am just trying to use my hardware's full capacity. Also please do not tell met to buy new hardware, I know I can use hardware amplification, I don't want to (like many others) spend money on a simple little problem like this.

Thanks!

Try a program like JACK, or possibly Virtual Audio Cable.

JACK works like a recording studio patch bay, allowing you to take audio input from one source or program and reroute it through other programs for processing before getting to audio hardware.

For your usage, you'd use JACK to route audio through some type of compressor plugin (VST or similar), that won't alter the volume of sounds over a certain threshold but will boost volumes of low-volume sounds.

I've never tried Virtual Audio Cable, but saw it recommended on another question and it sounds like it does basically the same thing as JACK.

This may not be quite what you're looking for (it works on audio files not arbitrary realtime audio) but take a look at Replay Gain.