We've found that the problem with really low volume in DivX movies is usually caused by the movies using the AC3 audio codec. The default volume for the AC3 codec is quite low, so raising the regular volume controls might not be enough for some movies. To fix the problem, simply goto the Start menu in Windows, select "Settings" and then the "Control Panel". If you have the AC3 codec installed on your system you should see the "AC3 Filter" listed there. Simply open the "AC3 Filter" and from there you should be able to change the Master volume, and even the volume of individual channels such as the Voice channel.
more divx movies troubleshooting: http://www.divxmovies.com/faq/