Audacity

Audacity is a free audio editor. It is free, as in price, and free as in “freedom of expression”.

Getting started with Audacity
Exporting an mp3
Volume
Panning
Making a sound story

Noteflight

With Noteflight you can compose notated music in your web browser. Noteflight is integrated with Ping.

Getting started with Noteflight
Clefs
Every Good Boy Deserves Fruit
Middle C
Quarter notes & rests
The Staff