Another possibility:
With a file loaded please check under:
(rightclick >) Options > Filters > Advanced
If you happen to see "ffdshow Audio Decoder" click on the small [+] and then click on "ffdshow audio" and select "Volume".
There you will find an option "Normalize".
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If it isn't ffdshow, but AC3 filter which you're seeing: I think it also has such option.