Equalizer APO - A system-wide equalizer for Windows 7 / 8 / 8.1 / 10 with channel remapping/copying capabilities
Ardour - Record, edit, and mix on Linux, Mac OS X, and Windows.
FxSound - Clearer Sound for Your PC. The new FxSound boosts your sound quality, volume, clarity and bass.
LMMS - Make music with a free, cross-platform tool
Boom 2 - Boom is a simple volume booster that lets you boost the volume of your Mac and your music files.
Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.