Get Avidemux.
Open your Video, under File menu > Information. Make a note of the Audio Codec, OK.
On the main screen Audio change Copy to your Audio Codec.
Hit Filters, Gain Mode > Manual.
Increase Gain Value (dB). Be careful how much you increase the dB. The value you increase is related to the original volume.
Save movie.
Occasionally I download some .avi files to view in WMP or DiVX format. Most of the time the volume is acceptable. But some files have audio that is so low I have to crank up my speaker volume all the way. Is there any software I can run these files through to increase the volume before playing?

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