Napr.
1) Load the .AVI file into VirtualDub.
2) Under the "Video" dropdown, change setting to "Direct Stream Copying".
3) Under the "Audio" dropdown, change the setting to "Full Processing Mode". This will make the "Volume" line active.
4) Set the volume to your desired percentage of the original.
5) Under "Audio", select "Compression". Select the same compression as was use for the original .AVI (e.g. .MP3). Put a checkmark in the box labeled "Show all formats". Select the same rate as was used for the original .AVI file (e.g. 44,100Hz Stereo 96Kbps)
6) Save as a new file name.

Alebo lepsie: vo virtualdub-e (alebo asi bude lepsi virtualdubmod) si z avi extrahujes zvukovu stopu, avi je univerzalny format takze zvuk moze byt v akomkolvek formate, ale vacsinou je v mp3 takze by z toho mohlo vyliezt nejake mp3, a to si mozes upravit programom napr. mp3gain (www.google.com) bez rekompresie (bez straty kvality). Potom upravenu audiostopu spojit s filmom vo virtualdubmod-e. P.S. netestoval som to prakticky takze je mozne ze ten postup nie je presny/uplny.