[all variants] Software to enhance sound?

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http://ubuntuforums.org – I have both mp3 and mp4 files with sound issues, there are two separate voices. 1 voice can be heard pretty well, the second voice is pretty quiet. Is there a software that can help me enhance the second quieter voice, or tone down the more prominent voice so that I can play the file and get both voices to be atleast fairly equal? (Hardware)