Audacity
How-To-Videos
https://www.smore.com/3j5hd
Audacity Basics
https://www.youtube.com/watch?v=DJhIlsLH32I
Audacity - Complete Tutorial
https://www.youtube.com/watch?v=aCisC3sHneM
Add Clips to One Project
Audacity Add Clips To One Project
Mixing Multiple Files in Audacity
Getting started with Audacity: Mixing multiple tracks
Adding Sound Effects in Audacity
Adding sound effects in Audacity
Adding background music or sound effects to recording
How to add background music in Audacity