Audacity (audio editor and recorder)
It's a free cross-platform sound editor. Mixing sounds, change speed or pitch of recording, resampling, dithering etc are few features of Audacity, which any sound editor provides.
File formats: You can work with .wav, .mp3, .mpeg and other file formats.
Sound quality: 16-bit, 24-bit and 32-bit recording and editing is possible. You can try with some provided sample files.
Plug-ins are also available to add special effects.
It's difficult to explain complete step by step process in this small section. But, here is the information for newbies: Editing for Beginners.
1) Open the Audacity software.
2) Open a .mp3 or any supported sound file format that you have.
3) You will see the wave signs of your opened sound file. Click on 'Play' button to hear that.
4) Below the menu bar, you will find some tools (basically, toolbar) using which you can select
the section of audio and Cut, Copy or Paste it in any new file OR in any other sound file (like, Mixing the audios). - visit 'how to cut, copy & paste...'
5) Generate, Effect and Analyze menus will provide the options like Split, Submix, Duplicate, Fade in, Fade out etc tools to play with sound files.
Screenshot of Audacity
Know more about Audacity.
Mostly, I used Sound Forge Studio but, the basic features / terminology will be same in all softwares to learn and operate it easily.
I hope this information will be helpful. Let me know if have any further questions!