Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Ashampoo® Soundstage Pro - Ashampoo Soundstage Pro processes all audio signals on your PC and alters them to simulate how they would sound on an actual surround system.
Noise Destroyer - Remove Backround Noise From Audio or Video
DarkAudacity - DarkAudacity is a build/fork of Audacity with quote "a darker more modern theme - and a few small tweaks."
Adobe Audition - Mix, edit, and create audio content in Adobe Audition CC with a comprehensive toolset that includes multitrack, waveform, and spectral display.
GarageBand - GarageBand is a fully equipped music creation studio for Mac