Audacity is a powerful open source audio editing program, with many high quality filters you an use to process you audio and make it sound exactly how you want it.
There have been changes in how this open source package in managed, including a controversy over data collection. As alternative you may consider Tenacity, which is a fork of this project
Available for Mac, PC and Linux
To make optimal use of Audacity (to read and write .mp3 for instance) you need to install ffmpeg