For mp3 it is mp3 gain, for wav there is wavegain (and ogggain for vorbis). But beware, wave gain is not fully lossless, you can loss a few quality as it makes integer rounding and it works only for n PCM wav which are never used with video. In fact wav container can contain many formats, so you will never find a tool that can level down or up the sound for all without a serious quality loss issue.
By the way there is ogggain too.
The only universal and perfectly lossless method is replaygain but it needs a special player so it won't work with virtualdub.
If it is because your videos have too quiet sound you can still use MatrixMuxer.