I have recently been watching hi definition video downloads on my laptop. Their sound was encoded using AC3 and I found that I was not able to get the sound loud enough on my laptop to watch the movie enjoyably.
The movies were mkv format (otherwise known as Matroska).
Fortunately, I found a very useful AC3 configuration too called the AC3 Filter that allows you to boost the sounds volume and push your speakers a little farther. You can download the program off their site and install it.
Just open it up, adjust the gain or any other settings, restart your media player, and enjoy. The only catch is that you will need to restart your media playing application anytime you change a setting to see its effect.