Me and me missus settled down to watch a movie (300) yesterday only to realize that the volume of the movie was considerably low. All controls on alsa mixer were jacked up to full and it wasn't related to any of the default volume level bugs; I had done an alsamixer -c0 to jack up the volume levels. I have only one audio card.
I searched on the interwebs for a software mixer program that could give me something a little more audible, no matter even if it was slightly screechy. I found nothing worthwhile and was about to give up. Then on a whim I opened up the movie on mplayer instead of the default totem (I rarely need to do that I must say) and started looking at what I could jack up there. Lo and behold, there was a slider in Preferences > Audio tab called Stereo boost (or something like that; it's on the other laptop and I'm too lazy to look it up now), which worked for me. I turned it all the way up and was finally able to understand the dialogue. It's a different thing that the movie turned out to be mostly about people cutting up other people (of course, that was fun too ;) ) so my efforts were worthwhile only for the first few minutes of the movie.
That said, this does not work for all kinds of movies since it seems to turn up the volume only on a specific channel. Due to this, on another movie, the dialogue was drowned out by traffic noise due to this stereo boost being jacked all the way up. Anyway, the point of the post was to say that there is in fact a way to (sort of) amplify sound in software.
Also, it looks like alsa does not quite seem to give the same kind of output that the audio subsystem on windows would give. Maybe this is specific only to certain types of cards.