Audio recording techniques and principles:
- capture: mics/ directionality/ gain/ proximity
- basic digital theory
Audio editing techniques and principles:
- diegetic/ non-diegetic
- Foley/ sound effects; soundtrack
Jamendo is a great free site for finding soundtrack audio to use in your projects.
You will need to sign up and then you can download music.
There are lots of different styles to choose from.
The Youtube audio library is a fantastic free resource for both music and sound effects.