VirtualDub help - Dialogs: Audio volume
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Dialogs:

Video filters
Video frame rate control
Video color depth
Video range
Video compression
Audio filters
Audio interleaving
Audio compression
Audio conversion
Audio volume
Capture settings
Capture preferences
Capture volume meter
Preferences

Permits volume adjustments to audio, to make the audio louder or softer.

Note VirtualDub hard clips when amplifying audio, so boosting the volume too much will result in distortion.
Note It is easier to make soft audio louder than to make loud audio softer -- if the source audio was so loud that it clipped during recording, introducing distortion, reducing volume after the fact won't remove the distortion. On the other hand, amplifying soft audio also amplifies quantization noise, which is particularly bad with 8-bit source. Try not to adjust volume more than you need to; it is better to record audio at the right volume in the first place.