DSP Plugins are - unfortunately- not always working.
I used "Enhancer", which (if it does work) does the trick quite capable.
However, you can also change the sound level in the video (.avi) file itself.
E.g. using VirtualDub
http://www.virtualdub.org/index
(Setting for video "Direct stream copy" and for audio "Full processing mode" and making use of the "Adjust volume of audio channels" option.)
I've "reworked" several videos in this way.