Use free program called Virtual Dub.
Under file, choose "file information" and see what the audio sampling rate is, normally, 22050.
Under Video, make sure direct streaming is chosen.
Under Audio, select Full Processing Mode (not direct stream) and then a volume option should appear. Here's where you can adjust volume higher (or lower).
Under Audio, slect "compression" and choose MPEG 3, and the bitrate that matches the sampling found in the file information, ie. 56kb 22,050HZ Stereo 7/KBs
If you don't compress the audio, it may skip during playback.
Next step you can preview the new louder sound by selecting "preview filtered" from the file menu.
If the volume sounds good, go ahead and choose "Save as AVI" from the file menu and save your new louder AVI file.