Hello,

It is possible to study music theory and sound thanks to scientific documents from acoustics (science of sound and mechanical waves).

Lots of documents are free online, as example :
On the Sensations of Tone as a Physiological Basis for the Theory of Music by H. Helmholtz
ou
Acoustics and the Performance of Music by Jürgen Meyer.

You may find lots of ressources on those website : Wikipedia, HAL et les Anna's Archive.

Lots of audio creation software (DAW) are free.

Most of controllers come with a lite version of a DAW (Digital Audio Workstation).

Virtual instrument comming with DAWs are often limiting, there are many VSTs available for free. Some are of excellent quality (many are listed on Plugin4Free).

Plenty of visual production software are free, however some can be complex to learn.

Some of the main examples :
Shotcut (video montage)
GIMP (picture edition)
Blender (VFX editor, 3D and others)

Here is a list of websites that provide resources as samples, images, videos or design:

BBC Sounds Effects
Freesound
Looperman
Archive
The Inspiration Grid
Hystory Maps